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b=HJazsxFUG4W4O32s3nlsSJSVqvdrptb8QHsDJhHF0d3dCy0/KXL/9ysgcpJIhGjHT XSpw5cWg9yTkcp6xcpszxbVsVgwxhC2gw+Eg3dTXhVO3v0JO9ZKDNYcOUlhrfEcXvc ACvEExtrMsCr1qtov9bnEWmN+qbv5845tsqmZw+E= Date: Thu, 25 Jul 2024 13:41:29 -0700 From: Andrew Morton To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , Peter Xu , Oscar Salvador Subject: Re: [PATCH v1 0/2] mm/hugetlb: fix hugetlb vs. core-mm PT locking Message-Id: <20240725134129.fc9165ac6413c4f774b786c1@linux-foundation.org> In-Reply-To: <20240725183955.2268884-1-david@redhat.com> References: <20240725183955.2268884-1-david@redhat.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CF720A0022 X-Stat-Signature: 4sjhdtug1e4aa5hcjdgjfiei1ribestc X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1721940094-244417 X-HE-Meta: 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 Ry9uUCRz hHhIWwURctn+uDMIY+4pZ/E6AbZAtPXQZbjvXRwcXSimd2VIYMst+ACCOJt0n+fBu6vHhC9OxsPjiFRZ9ZQP+xbvH3SfotboC1/IfK2o5OdtWF0/rw5tRWQYsVEp43pqrzV0w/GzFElank/eWtq+ukKPhNv/rCb936yFBxqOBkdYRYz7+OnGcJwn+YoAdmut02xNWyhctp8QkI9Hyd41j1rLB6Nbbmi94gpzwWasNXHXZX7Eg8FPgDhaSzg== 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: List-Subscribe: List-Unsubscribe: On Thu, 25 Jul 2024 20:39:53 +0200 David Hildenbrand wrote: > Working on another generic page table walker that tries to avoid > special-casing hugetlb, I found a page table locking issue with hugetlb > folios that are not mapped using a single PMD/PUD. > > For some hugetlb folio sizes, GUP will take different page table locks > when walking the page tables than hugetlb when modifying the page tables. > > I did not actually try reproducing an issue, but looking at > follow_pmd_mask() where we might be rereading a PMD value multiple times > it's rather clear that concurrent modifications are rather unpleasant. > > In follow_page_pte() we might be better in that regard -- ptep_get() does > a READ_ONCE() -- but who knows what else could happen concurrently in > some weird corner cases (e.g., hugetlb folio getting unmapped and freed). > > Did some basic sanity testing with various hugetlb sizes on x86-64 and > arm64. Maybe I'll find some time to actually write a simple reproducer in > the common weeks, so this wouldn't have to be all-theoretical for now. When can we be confident that this change is merge-worthy? > Only v6.10 is affected, so the #1 can be simply backported as a prereq > patch along with the real fix. I'll add the same Fixes: to [1/2], and cc:stable.