Re: [PATCH] mm, page_alloc: Always use a captured page regardless of compaction result

[Vlastimil Babka]

On 4/19/19 10:51 AM, Mel Gorman wrote:
> 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.
> 
> [ 1422.124060] WARNING: CPU: 0 PID: 9783 at mm/page_alloc.c:3777 __alloc_pages_direct_compact+0x182/0x190
> [ 1422.124065] 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
> [ 1422.124086] 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.
> 
> Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
> Reported-by: Li Wang <liwang@xxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Ah, noticed the new formal resend only after replying to the first one,
so here goes again:


Yup, no need for a Cc: stable on a very rare WARN_ON_ONCE. So the AI
will pick it anyway...

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>


> ---
>  mm/page_alloc.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d96ca5bc555b..cfaba3889fa2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3773,11 +3773,6 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  	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
>