From: Hugh Dickins <hughd@google.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Greg Thelen <gthelen@google.com>, Hugh Dickins <hughd@google.com>,
David Rientjes <rientjes@google.com>,
Vlastimil Babka <vbabka@suse.cz>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
Rik van Riel <riel@redhat.com>, Christoph Lameter <cl@gentwo.org>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Steve Capper <steve.capper@linaro.org>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@suse.cz>,
Jerome Marchand <jmarchan@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: page-flags behavior on compound pages: a worry
Date: Wed, 12 Aug 2015 21:12:07 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.11.1508122038380.4539@eggly.anvils> (raw)
In-Reply-To: <20150812222136.GA15010@node.dhcp.inet.fi>
On Thu, 13 Aug 2015, Kirill A. Shutemov wrote:
>
> All this situation is ugly. I'm thinking on more general solution for
> PageTail() vs. ->first_page race.
>
> We would be able to avoid the race in first place if we encode PageTail()
> and position of head page within the same word in struct page. This way we
> update both thing in one shot without possibility of race.
>
> Details get tricky.
>
> I'm going to try tomorrow something like this: encode the position of head
> as offset from the tail page and store it as negative number in the union
> with ->mapping and ->s_mem. PageTail() can be implemented as check value
> of the field to be in range -1..-MAX_ORDER_NR_PAGES.
>
> I'm not sure at all if it's going to work, especially looking on
> ridiculously high CONFIG_FORCE_MAX_ZONEORDER some architectures allow.
>
> We could also try to encode page order instead (again as negative number)
> and calculate head page position based on alignment...
>
> Any other ideas are welcome.
Good luck, I've not given it any thought, but hope it works out:
my reasoning was the same when I put the PageAnon bit into
page->mapping instead of page->flags.
Something to beware of though: although exceedingly unlikely to be a
problem, page->mapping always contained a pointer to or into a relevant
structure, or else something that could not possibly be a kernel pointer,
when I was working on KSM swapping: see comment above get_ksm_page() in
mm/ksm.c. It is best to keep page->mapping for pointers if possible
(and probably avoid having the PageAnon bit set unless really Anon).
I've only just read your mail, and I'm too slow a thinker to have
worked through your isolate_migratepages_block() race yet. But, given
the timing, cannot resist sending you a code fragment I wrote earlier
today for our v3.11-based kernel: which still has compound_trans_order(),
which we had been using in a similar racy physical scan.
I'm not for a moment suggesting that this fragment is relevant to your
race; but it is something amusing to consider when you're thinking of
such races. Credit to Greg Thelen for thinking of the prep_compound_page()
end of it, when I'd been focussed on the __split_huge_page_refcount() end.
/*
* It is not safe to use compound_lock (inside compound_trans_order)
* until we have a reference on the page (okay, done above) and have
* then seen PageLRU on it (just below): because mm/huge_memory.c uses
* the non-atomic __SetPageUptodate on a freshly allocated THPage in
* several places, believing it to be invisible to the outside world,
* but liable to race and leave PG_compound_lock set when cleared here.
*/
nr_pages = 1;
if (PageHead(page)) {
/*
* smp_rmb() against the smp_wmb() in the first iteration of
* prep_compound_page(), so that the PageTail test ensures
* that compound_order(page) is now correctly readable.
*/
smp_rmb();
if (PageTail(page + 1)) {
nr_pages = 1 << compound_order(page);
/*
* Then smp_rmb() against smp_wmb() in last iteration of
* __split_huge_page_refcount(), to ensure that has not
* yet written something else into page[1].lru.prev.
*/
smp_rmb();
if (!PageTail(page + 1))
nr_pages = 1;
}
}
Hugh
next prev parent reply other threads:[~2015-08-13 4:13 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-19 17:08 [PATCH 00/16] Sanitize usage of ->flags and ->mapping for tail pages Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 01/16] mm: consolidate all page-flags helpers in <linux/page-flags.h> Kirill A. Shutemov
2015-03-23 0:10 ` Hugh Dickins
2015-03-19 17:08 ` [PATCH 02/16] page-flags: trivial cleanup for PageTrans* helpers Kirill A. Shutemov
2015-03-23 0:12 ` Hugh Dickins
2015-03-19 17:08 ` [PATCH 03/16] page-flags: introduce page flags policies wrt compound pages Kirill A. Shutemov
2015-03-20 20:35 ` Andrew Morton
2015-03-20 21:34 ` Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 04/16] page-flags: define PG_locked behavior on " Kirill A. Shutemov
2015-03-27 15:11 ` Mateusz Krawczuk
2015-03-27 15:13 ` Mateusz Krawczuk
2015-03-27 16:37 ` Kirill A. Shutemov
2015-07-15 20:20 ` Christoph Lameter
2015-08-06 4:15 ` page-flags behavior on compound pages: a worry Hugh Dickins
2015-08-06 15:33 ` Kirill A. Shutemov
2015-08-06 19:24 ` Hugh Dickins
2015-08-06 20:45 ` Christoph Lameter
2015-08-07 14:50 ` Kirill A. Shutemov
2015-08-07 15:28 ` Christoph Lameter
2015-08-10 11:09 ` Kirill A. Shutemov
2015-08-10 13:50 ` Christoph Lameter
2015-08-07 14:49 ` Kirill A. Shutemov
2015-08-13 5:10 ` Hugh Dickins
2015-08-12 14:35 ` Kirill A. Shutemov
2015-08-12 14:47 ` Vlastimil Babka
2015-08-12 21:16 ` Andrew Morton
2015-08-12 22:21 ` Kirill A. Shutemov
2015-08-13 4:12 ` Hugh Dickins [this message]
2015-03-19 17:08 ` [PATCH 05/16] page-flags: define behavior of FS/IO-related flags on compound pages Kirill A. Shutemov
2015-03-19 18:29 ` Dave Hansen
2015-03-19 20:02 ` Kirill A. Shutemov
2015-03-23 0:02 ` Hugh Dickins
2015-03-23 12:17 ` Kirill A. Shutemov
2015-03-24 22:54 ` Hugh Dickins
2015-03-25 10:23 ` Kirill A. Shutemov
2015-03-25 18:56 ` Hugh Dickins
2015-03-19 17:08 ` [PATCH 06/16] page-flags: define behavior of LRU-related " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 07/16] page-flags: define behavior SL*B-related " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 08/16] page-flags: define behavior of Xen-related " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 09/16] page-flags: define PG_reserved behavior " Kirill A. Shutemov
2020-01-31 15:24 ` Chris Wilson
2020-02-03 15:18 ` Kirill A. Shutemov
2020-02-03 15:24 ` Chris Wilson
2020-02-03 17:10 ` David Hildenbrand
2020-02-03 17:29 ` Christoph Hellwig
2015-03-19 17:08 ` [PATCH 10/16] page-flags: define PG_swapbacked " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 11/16] page-flags: define PG_swapcache " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 12/16] page-flags: define PG_mlocked " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 13/16] page-flags: define PG_uncached " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 14/16] page-flags: define PG_uptodate " Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 15/16] page-flags: look on head page if the flag is encoded in page->mapping Kirill A. Shutemov
2015-03-19 17:08 ` [PATCH 16/16] mm: sanitize page->mapping for tail pages Kirill A. Shutemov
2015-03-23 0:28 ` [PATCH 00/16] Sanitize usage of ->flags and ->mapping " Hugh Dickins
2015-03-23 10:04 ` Kirill A. Shutemov
2015-03-24 23:42 ` Hugh Dickins
2015-03-25 10:55 ` Kirill A. Shutemov
2015-03-24 17:39 ` Konstantin Khlebnikov
2015-03-24 20:04 ` Kirill A. Shutemov
2015-07-15 20:20 ` Christoph Lameter
2015-07-15 21:18 ` Kirill A. Shutemov
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