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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id m66sm133009oia.28.2021.05.26.15.48.55 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Wed, 26 May 2021 15:48:55 -0700 (PDT) Date: Wed, 26 May 2021 15:48:42 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Yang Shi cc: Hugh Dickins , Zi Yan , "Kirill A. Shutemov" , =?UTF-8?Q?HORIGUCHI_NAOYA=28=E5=A0=80=E5=8F=A3_=E7=9B=B4=E4=B9=9F=29?= , Wang Yugui , Andrew Morton , Linux MM , Linux Kernel Mailing List Subject: Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split In-Reply-To: Message-ID: References: <20210525162145.3510-1-shy828301@gmail.com> <20210525162145.3510-2-shy828301@gmail.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=shqvBt5S; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf06.hostedemail.com: domain of hughd@google.com designates 209.85.210.42 as permitted sender) smtp.mailfrom=hughd@google.com X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 25841C0042C4 X-Stat-Signature: w8pzupokq149q13qspewe3gs9u64a5z6 X-HE-Tag: 1622069330-450472 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 26 May 2021, Yang Shi wrote: > On Tue, May 25, 2021 at 4:58 PM Hugh Dickins wrote: > > On Tue, 25 May 2021, Yang Shi wrote: > > > > > > We should be able to make dump_page() print total mapcount, right? The > > > dump_page() should be just called in some error paths so taking some > > > extra overhead to dump more information seems harmless, or am I > > > missing something? Of course, this can be done in a separate patch. > > > > I didn't want to ask that of you, but yes, if you're willing to add > > total_mapcount() into dump_page(), I think that would be ideal; and > > could be helpful for other cases too. > > > > Looking through total_mapcount(), I think it's safe to call from > > dump_page() - I always worry about extending crash info with > > something that depends on a maybe-corrupted pointer which would > > generate a further crash and either recurse or truncate the output - > > but please check that carefully. > > Yes, it is possible. If the THP is being split, some VM_BUG_* might be > triggered if total_mapcount() is called. But it is still feasible to > print total mapcount as long as we implement a more robust version for > dump_page(). Oh dear. I think the very last thing the kernel needs is yet another subtly different variant of *mapcount*(). Do you have a specific VM_BUG_* in mind there? Of course there's the VM_BUG_ON_PAGE(PageTail) at the start of it, and you'd want to print total_mapcount(head) to avoid that one. Looks like __dump_page() is already careful about "head", checking whether "page" is within the expected bounds. Of course, once we're in serious VM_WARN territory, there might be races which could flip fields midway: PageTail set by the time it reaches total_mapcount()? Narrow the race (rather like it does with PageSlab) by testing PageTail immediately before calling total_mapcount(head)? Hugh