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Since a28334862993 ("page cache: Finish XArray conversion"), on most
major Linux distributions, the page cache doesn't correctly transition
when the hot data set is changing, and leaves the new pages thrashing
indefinitely instead of kicking out the cold ones.
On a freshly booted, freshly ssh'd into virtual machine with 1G RAM
running stock Arch Linux:
[root@ham ~]# ./reclaimtest.sh
+ dd of=workingset-a bs=1M count=0 seek=600
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ ./mincore workingset-a
153600/153600 workingset-a
+ dd of=workingset-b bs=1M count=0 seek=600
+ cat workingset-b
+ cat workingset-b
+ cat workingset-b
+ cat workingset-b
+ ./mincore workingset-a workingset-b
104029/153600 workingset-a
120086/153600 workingset-b
+ cat workingset-b
+ cat workingset-b
+ cat workingset-b
+ cat workingset-b
+ ./mincore workingset-a workingset-b
104029/153600 workingset-a
120268/153600 workingset-b
workingset-b is a 600M file on a 1G host that is otherwise entirely
idle. No matter how often it's being accessed, it won't get cached.
While investigating, I noticed that the non-resident information gets
aggressively reclaimed - /proc/vmstat::workingset_nodereclaim. This is
a problem because a workingset transition like this relies on the
non-resident information tracked in the page cache tree of evicted
file ranges: when the cache faults are refaults of recently evicted
cache, we challenge the existing active set, and that allows a new
workingset to establish itself.
Tracing the shrinker that maintains this memory revealed that all page
cache tree nodes were allocated to the root cgroup. This is a problem,
because 1) the shrinker sizes the amount of non-resident information
it keeps to the size of the cgroup's other memory and 2) on most major
Linux distributions, only kernel threads live in the root cgroup and
everything else gets put into services or session groups:
[root@ham ~]# cat /proc/self/cgroup
0::/user.slice/user-0.slice/session-c1.scope
As a result, we basically maintain no non-resident information for the
workloads running on the system, thus breaking the caching algorithm.
Looking through the code, I found the culprit in the above-mentioned
patch: when switching from the radix tree to xarray, it dropped the
__GFP_ACCOUNT flag from the tree node allocations - the flag that
makes sure the allocated memory gets charged to and tracked by the
cgroup of the calling process - in this case, the one doing the fault.
To fix this, allow xarray users to specify per-tree flag that makes
xarray allocate nodes using __GFP_ACCOUNT. Then restore the page cache
tree annotation to request such cgroup tracking for the cache nodes.
With this patch applied, the page cache correctly converges on new
workingsets again after just a few iterations:
[root@ham ~]# ./reclaimtest.sh
+ dd of=workingset-a bs=1M count=0 seek=600
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ cat workingset-a
+ ./mincore workingset-a
153600/153600 workingset-a
+ dd of=workingset-b bs=1M count=0 seek=600
+ cat workingset-b
+ ./mincore workingset-a workingset-b
124607/153600 workingset-a
87876/153600 workingset-b
+ cat workingset-b
+ ./mincore workingset-a workingset-b
81313/153600 workingset-a
133321/153600 workingset-b
+ cat workingset-b
+ ./mincore workingset-a workingset-b
63036/153600 workingset-a
153600/153600 workingset-b
Cc: stable@vger.kernel.org # 4.20+
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
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If we reserve index 0, the next entry to be stored there might be 2-byte
aligned. That means we have to create the root xa_node at the time of
reserving the initial entry.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xas_store() was interpreting the entry it found in the array as a node
entry if the bottom two bits had value 2. That's only true if either
the entry is in the root node or in a non-leaf node.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Jason feels this is clearer, and it saves a function and an exported
symbol.
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xa_cmpxchg() was a little too magic in turning ZERO entries into NULL,
and would leave the entry set to the ZERO entry instead of releasing
it for future use. After careful review of existing users of
xa_cmpxchg(), change the semantics so that it does not translate either
incoming argument from NULL into ZERO entries.
Add several tests to the test-suite to make sure this problem doesn't
come back.
Reported-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This differs slightly from the IDR equivalent in five ways.
1. It can allocate up to UINT_MAX instead of being limited to INT_MAX,
like xa_alloc(). Also like xa_alloc(), it will write to the 'id'
pointer before placing the entry in the XArray.
2. The 'next' cursor is allocated separately from the XArray instead
of being part of the IDR. This saves memory for all the users which
do not use the cyclic allocation API and suits some users better.
3. It returns -EBUSY instead of -ENOSPC.
4. It will attempt to wrap back to the minimum value on memory allocation
failure as well as on an -EBUSY error, assuming that a user would
rather allocate a small ID than suffer an ID allocation failure.
5. It reports whether it has wrapped, which is important to some users.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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It was too easy to forget to initialise the start index. Add an
xa_limit data structure which can be used to pass min & max, and
define a couple of special values for common cases. Also add some
more tests cribbed from the IDR test suite. Change the return value
from -ENOSPC to -EBUSY to match xa_insert().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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A lot of places want to allocate IDs starting at 1 instead of 0.
While the xa_alloc() API supports this, it's not very efficient if lots
of IDs are allocated, due to having to walk down to the bottom of the
tree to see if ID 1 is available, then all the way over to the next
non-allocated ID. This method marks ID 0 as being occupied which wastes
one slot in the XArray, but preserves xa_empty() as working.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Userspace translates EEXIST to "File exists" which isn't a very good
error message for the problem. "Device or resource busy" is a better
indication of what went wrong.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xa_erase does not allocate memory and doesn't have a gfp parameter.
Update the descriptions of all four variants to be more useful.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xa_insert() should treat reserved entries as occupied, not as available.
Also, it should treat requests to insert a NULL pointer as a request
to reserve the slot. Add xa_insert_bh() and xa_insert_irq() for
completeness.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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On m68k, statically allocated pointers may only be two-byte aligned.
This clashes with the XArray's method for tagging internal pointers.
Permit storing these pointers in single slots (ie not in multislots).
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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A regular xa_init_flags() put all dynamically-initialised XArrays into
the same locking class. That leads to lockdep believing that taking
one XArray lock while holding another is a deadlock. It's possible to
work around some of these situations with separate locking classes for
irq/bh/regular XArrays, and SINGLE_DEPTH_NESTING, but that's ugly, and
it doesn't work for all situations (where we have completely unrelated
XArrays).
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Specifying a starting ID greater than the maximum ID isn't something
attempted very often, but it should fail. It was succeeding due to
xas_find_marked() returning the wrong error state, so add tests for
both xa_alloc() and xas_find_marked().
Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The explicit '64' should have been BITS_PER_LONG, but while looking at
this code I realised I meant to use __ffs(), not ilog2().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Minor fixes.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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For allocating XArrays, it makes sense to distinguish beteen erasing an
entry and storing NULL. Storing NULL keeps the index allocated with a
NULL pointer associated with it while xa_erase() frees the index. Some
existing IDR users rely on this ability.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Saves around 115 bytes on a tinyconfig build and reduces the amount
of code duplication in the XArray implementation.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Make xa_erase() take the spinlock and then call __xa_erase(), but make
it out of line since it's such a common function.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xa_cmpxchg() was one of the largest functions in the xarray
implementation. By turning it into a wrapper and having the callers
take the lock (like several other functions), we save 160 bytes on a
tinyconfig build and reduce the duplication in xarray.c.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The xa_reserve() function was a little unusual in that it attempted to
be callable for all kinds of locking scenarios. Make it look like the
other APIs with __xa_reserve, xa_reserve_bh and xa_reserve_irq variants.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Without this, it's not possible to use static inlines like xa_store_bh()
and xa_erase_irq().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The following sequence of calls would result in an infinite loop in
xa_find_after():
xa_store(xa, 0, x, GFP_KERNEL);
index = 0;
xa_for_each(xa, entry, index, ULONG_MAX, XA_PRESENT) { }
xa_find_after() was confusing the situation where we found no entry in
the tree with finding a multiorder entry, so it would look for the
successor entry forever. Just check for this case explicitly. Includes
a few new checks in the test suite to be sure this doesn't reappear.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This version of xa_store_range() really only supports load and store.
Our only user only needs basic load and store functionality, so there's
no need to do the extra work to support marking and overlapping stores
correctly yet.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Add the optional ability to track which entries in an XArray are free
and provide xa_alloc() to replace most of the functionality of the IDR.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This function reserves a slot in the XArray for users which need
to acquire multiple locks before storing their entry in the tree and
so cannot use a plain xa_store().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This hopefully temporary function is useful for users who have not yet
been converted to multi-index entries.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This iterator iterates over each entry that is stored in the index or
indices specified by the xa_state. This is intended for use for a
conditional store of a multiindex entry, or to allow entries which are
about to be removed from the xarray to be disposed of properly.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The xas_next and xas_prev functions move the xas index by one position,
and adjust the rest of the iterator state to match it. This is more
efficient than calling xas_set() as it keeps the iterator at the leaves
of the tree instead of walking the iterator from the root each time.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This function frees all the internal memory allocated to the xarray
and reinitialises it to be empty.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The xa_extract function combines the functionality of
radix_tree_gang_lookup() and radix_tree_gang_lookup_tagged().
It extracts entries matching the specified filter into a normal array.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The xa_for_each iterator allows the user to efficiently walk a range
of the array, executing the loop body once for each entry in that
range that matches the filter. This commit also includes xa_find()
and xa_find_after() which are helper functions for xa_for_each() but
may also be useful in their own right.
In the xas family of functions, we have xas_for_each(), xas_find(),
xas_next_entry(), xas_for_each_tagged(), xas_find_tagged(),
xas_next_tagged() and xas_pause().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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Like cmpxchg(), xa_cmpxchg will only store to the index if the current
entry matches the old entry. It returns the current entry, which is
usually more useful than the errno returned by radix_tree_insert().
For the users who really only want the errno, the xa_insert() wrapper
provides a more convenient calling convention.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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xa_store() differs from radix_tree_insert() in that it will overwrite an
existing element in the array rather than returning an error. This is
the behaviour which most users want, and those that want more complex
behaviour generally want to use the xas family of routines anyway.
For memory allocation, xa_store() will first attempt to request memory
from the slab allocator; if memory is not immediately available, it will
drop the xa_lock and allocate memory, keeping a pointer in the xa_state.
It does not use the per-CPU cache, although those will continue to exist
until all radix tree users are converted to the xarray.
This patch also includes xa_erase() and __xa_erase() for a streamlined
way to store NULL. Since there is no need to allocate memory in order
to store a NULL in the XArray, we do not need to trouble the user with
deciding what memory allocation flags to use.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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XArray marks are like the radix tree tags, only slightly more strongly
typed. They are renamed in order to distinguish them from tagged
pointers. This commit adds the basic get/set/clear operations.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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The xa_load function brings with it a lot of infrastructure; xa_empty(),
xa_is_err(), and large chunks of the XArray advanced API that are used
to implement xa_load.
As the test-suite demonstrates, it is possible to use the XArray functions
on a radix tree. The radix tree functions depend on the GFP flags being
stored in the root of the tree, so it's not possible to use the radix
tree functions on an XArray.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
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This is a direct replacement for struct radix_tree_root. Some of the
struct members have changed name; convert those, and use a #define so
that radix_tree users continue to work without change.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>
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