[PATCH v4] Resolve LRU page-pinning issue for file-backed pages

[Chris Goldsworthy <cgoldswo@codeaurora.org>]

It is possible for file-backed pages to end up in a contiguous memory area
(CMA), such that the relevant page must be migrated using the .migratepage()
callback when its backing physical memory is selected for use in an CMA
allocation (through cma_alloc()).  However, if a set of address space
operations (AOPs) for a file-backed page lacks a migratepage() page call-back,
fallback_migrate_page() will be used instead, which through
try_to_release_page() calls try_to_free_buffers() (which is called directly or
through a try_to_free_buffers() callback.  try_to_free_buffers() in turn calls
drop_buffers()

drop_buffers() itself can fail due to the buffer_head associated with a page
being busy. However, it is possible that the buffer_head is on an LRU list for
a CPU, such that we can try removing the buffer_head from that list, in order
to successfully release the page.  Do this.

v1: https://lore.kernel.org/lkml/cover.1606194703.git.cgoldswo@codeaurora.org/T/#m3a44b5745054206665455625ccaf27379df8a190
Original version of the patch (with updates to make to account for changes in
on_each_cpu_cond()).

v2: https://lore.kernel.org/lkml/cover.1609829465.git.cgoldswo@codeaurora.org/
Follow Matthew Wilcox's suggestion of reducing the number of calls to
on_each_cpu_cond(), by iterating over a page's busy buffer_heads inside of
on_each_cpu_cond(). To copy from his e-mail, we go from:

for_each_buffer
	for_each_cpu
		for_each_lru_entry

to:

for_each_cpu
	for_each_buffer
		for_each_lru_entry

This is done using xarrays, which I found to be the cleanest data structure to
use, though a pre-allocated array of page_size(page) / bh->b_size elements might
be more performant.

v3: Replace xas_for_each() with xa_for_each() to account for proper locking.

v4: Fix an iteration error.

Laura Abbott (1):
  fs/buffer.c: Revoke LRU when trying to drop buffers

 fs/buffer.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 74 insertions(+), 5 deletions(-)

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