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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Peter Xu , linux-mm@kvack.org, Alex Williamson , Max Boone , stable@vger.kernel.org Subject: Re: [PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start() Message-ID: References: <20260323-follow_pfnmap_fix-v1-1-5b0ec10872b3@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260323-follow_pfnmap_fix-v1-1-5b0ec10872b3@kernel.org> X-Rspamd-Server: rspam12 X-Stat-Signature: 4pijyx871mwfeta1dpdf8b4wxd5suq44 X-Rspamd-Queue-Id: 9A960C0004 X-Rspam-User: X-HE-Tag: 1774341574-273861 X-HE-Meta: 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 eQhdZFpM PgaA8qJh7KOcXhZs0dpjn8bww8jX0kD8Vibrvtc1Omiof4XSNw/GZNmo2QppJ6rUle1kJq9FBvc9SplbNvH0w7a3Aq5wXpXkQTY9eyce8+OjudATYSN850uu8FK4/Zdq58mE/0NAK60wBk1Y0PEOK7Je8j4DGH1WT0bbLZnMcCENQzSE24G080/DR3UJQXpNQPNHgiZ+ujiDUChnZ1Mn0vGoUVvPH8U/ctMBUiHk2tUgGhJ/hWN5rkFBqM92ZGnBbvHNunG3Z2EGlit3/a9vuW6g5vSm1pyMjcUt6v1l5Tbd5O01Dn6mO9gk1V5AeBXyq3CHxM+blOIGKmD8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Mar 23, 2026 at 09:20:18PM +0100, David Hildenbrand (Arm) wrote: > follow_pfnmap_start() suffers from two problems: > > (1) We are not re-fetching the pmd/pud after taking the PTL > > Therefore, we are not properly stabilizing what the lock lock actually ^ lock lock > protects. If there is concurrent zapping, we would indicate to the > caller that we found an entry, however, that entry might already have > been invalidated, or contain a different PFN after taking the lock. > > Properly use pmdp_get() / pudp_get() after taking the lock. > > (2) pmd_leaf() / pud_leaf() are not well defined on non-present entries > > pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries. > > There is no real guarantee that pmd_leaf()/pud_leaf() returns something > reasonable on non-present entries. Most architectures indeed either > perform a present check or make it work by smart use of flags. > > However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(), > and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby > pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not > do that. > > Let's check pmd_present()/pud_present() before assuming "the is a > present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page > table handling code that traverses user page tables does. > > Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, > (1) is likely more relevant than (2). It is questionable how often (1) > would actually trigger, but let's CC stable to be sure. > > This was found by code inspection. > > Fixes: 6da8e9634bb7 ("mm: new follow_pfnmap API") > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) Acked-by: Mike Rapoport (Microsoft) > --- > Gave it a quick test in a VM with MM selftests etc, but I am not sure if > I actually trigger the follow_pfnmap machinery. Most probably not :) KVM selftests might, didn't really dig into that. But I doubt any selftest would trigger potential races there. > --- > mm/memory.c | 18 +++++++++++++++--- > 1 file changed, 15 insertions(+), 3 deletions(-) -- Sincerely yours, Mike.