linux-kernel - Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150521135242.GY2462@suse.de>
Date:	Thu, 21 May 2015 14:52:42 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE_PAGES

On Thu, May 21, 2015 at 03:27:22PM +0200, Michal Hocko wrote:
> hugetlb pages uses add_to_page_cache to track shared mappings. This
> is OK from the data structure point of view but it is less so from the
> NR_FILE_PAGES accounting:
> 	- huge pages are accounted as 4k which is clearly wrong
> 	- this counter is used as the amount of the reclaimable page
> 	  cache which is incorrect as well because hugetlb pages are
> 	  special and not reclaimable
> 	- the counter is then exported to userspace via /proc/meminfo
> 	  (in Cached:), /proc/vmstat and /proc/zoneinfo as
> 	  nr_file_pages which is confusing at least:
> 	  Cached:          8883504 kB
> 	  HugePages_Free:     8348
> 	  ...
> 	  Cached:          8916048 kB
> 	  HugePages_Free:      156
> 	  ...
> 	  thats 8192 huge pages allocated which is ~16G accounted as 32M
> 
> There are usually not that many huge pages in the system for this to
> make any visible difference e.g. by fooling __vm_enough_memory or
> zone_pagecache_reclaimable.
> 
> Fix this by special casing huge pages in both __delete_from_page_cache
> and __add_to_page_cache_locked. replace_page_cache_page is currently
> only used by fuse and that shouldn't touch hugetlb pages AFAICS but it
> is more robust to check for special casing there as well.
> 
> Hugetlb pages shouldn't get to any other paths where we do accounting:
> 	- migration - we have a special handling via
> 	  hugetlbfs_migrate_page
> 	- shmem - doesn't handle hugetlb pages directly even for
> 	  SHM_HUGETLB resp. MAP_HUGETLB
> 	- swapcache - hugetlb is not swapable
> 
> This has a user visible effect but I believe it is reasonable because
> the previously exported number is simply bogus.
> 
> An alternative would be to account hugetlb pages with their real size
> and treat them similar to shmem. But this has some drawbacks.
> 
> First we would have to special case in kernel users of NR_FILE_PAGES and
> considering how hugetlb is special we would have to do it everywhere. We
> do not want Cached exported by /proc/meminfo to include it because the
> value would be even more misleading.
> __vm_enough_memory and zone_pagecache_reclaimable would have to do
> the same thing because those pages are simply not reclaimable. The
> correction is even not trivial because we would have to consider all
> active hugetlb page sizes properly. Users of the counter outside of the
> kernel would have to do the same.
> So the question is why to account something that needs to be basically
> excluded for each reasonable usage. This doesn't make much sense to me.
> 
> It seems that this has been broken since hugetlb was introduced but I
> haven't checked the whole history.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/