mm, page_alloc: always use a captured page regardless of compaction result [Linux 5.1]

mm, page_alloc: always use a captured page regardless of compaction result [Linux 5.1]

This Linux kernel change "mm, page_alloc: always use a captured page regardless of compaction result" is included in the Linux 5.1 release. This change is authored by Mel Gorman <mgorman [at] techsingularity.net> on Thu Apr 25 22:23:54 2019 -0700. The commit for this change in Linux stable tree is ee8ab0e (patch).

mm, page_alloc: always use a captured page regardless of compaction result

During the development of commit 5e1f0f098b46 ("mm, compaction: capture
a page under direct compaction"), a paranoid check was added to ensure
that if a captured page was available after compaction that it was
consistent with the final state of compaction.  The intent was to catch
serious programming bugs such as using a stale page pointer and causing
corruption problems.

However, it is possible to get a captured page even if compaction was
unsuccessful if an interrupt triggered and happened to free pages in
interrupt context that got merged into a suitable high-order page.  It's
highly unlikely but Li Wang did report the following warning on s390
occuring when testing OOM handling.  Note that the warning is slightly
edited for clarity.

  WARNING: CPU: 0 PID: 9783 at mm/page_alloc.c:3777 __alloc_pages_direct_compact+0x182/0x190
  Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver nfs
    lockd grace fscache sunrpc pkey ghash_s390 prng xts aes_s390
    des_s390 des_generic sha512_s390 zcrypt_cex4 zcrypt vmur binfmt_misc
    ip_tables xfs libcrc32c dasd_fba_mod qeth_l2 dasd_eckd_mod dasd_mod
    qeth qdio lcs ctcm ccwgroup fsm dm_mirror dm_region_hash dm_log
    dm_mod
  CPU: 0 PID: 9783 Comm: copy.sh Kdump: loaded Not tainted 5.1.0-rc 5 #1

This patch simply removes the check entirely instead of trying to be
clever about pages freed from interrupt context.  If a serious
programming error was introduced, it is highly likely to be caught by
prep_new_page() instead.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
Signed-off-by: Mel Gorman <[email protected]>
Reported-by: Li Wang <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

There are 5 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 mm/page_alloc.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6c6d9f1..d167c48 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3786,11 +3786,6 @@ void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
    memalloc_noreclaim_restore(noreclaim_flag);
    psi_memstall_leave(&pflags);

-   if (*compact_result <= COMPACT_INACTIVE) {
-       WARN_ON_ONCE(page);
-       return NULL;
-   }
-
    /*
     * At least in one zone compaction wasn't deferred or skipped, so let's
     * count a compaction stall

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