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Wed, 18 Mar 2026 18:12:48 -0700 (PDT) From: Jiakai Xu X-Google-Original-From: Jiakai Xu To: ljs@kernel.org Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, david@kernel.org, harry.yoo@oracle.com, jannh@google.com, jiakaipeanut@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, riel@surriel.com, sashal@kernel.org, vbabka@kernel.org, Jiakai Xu Subject: Re: [PATCH mm-hotfixes] mm/rmap: clear vma->anon_vma on error Date: Thu, 19 Mar 2026 09:12:29 +0800 Message-Id: <20260319011229.602-1-jiakaiPeanut@gmail.com> X-Mailer: git-send-email 2.35.1.windows.2 In-Reply-To: <20260318122632.63404-1-ljs@kernel.org> References: <20260318122632.63404-1-ljs@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable > Commit 542eda1a8329 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas= ()=0D > comments, add asserts") alters the way errors are handled, but overlooked= =0D > one important aspect of clean up.=0D > =0D > When a VMA encounters an error state in anon_vma_clone() (that is, on=0D > attempted allocation of anon_vma_chain objects), it cleans up partially=0D > established state in cleanup_partial_anon_vmas(), before returning an=0D > error.=0D > =0D > However, this occurs prior to anon_vma->num_active_vmas being incremented= ,=0D > and it also fails to clear the VMA's vma->anon_vma field, which remains i= n=0D > place.=0D > =0D > This is immediately an inconsistent state, because=0D > anon_vma->num_active_vmas is supposed to track the number of VMAs whose=0D > vma->anon_vma field references that anon_vma, and now that count is=0D > off-by-negative-1 for each VMA for which this error state has occurred.=0D > =0D > When VMAs are unlinked from this anon_vma, unlink_anon_vmas() will=0D > eventually underflow anon_vma->num_active_vmas, which will trigger a=0D > warning.=0D > =0D > This will always eventually happen, as we unlink anon_vma's at process=0D > teardown.=0D > =0D > It could also cause maybe_reuse_anon_vma() to incorrectly permit the reus= e=0D > of an anon_vma which has active VMAs attached, which will lead to a=0D > persistently invalid state.=0D > =0D > The solution is to clear the VMA's anon_vma field when we clean up partia= l=0D > state, as the fact we are doing so indicates clearly that the VMA is not= =0D > correctly integrated into the anon_vma tree and thus this field is invali= d.=0D > =0D > Reported-by: Sasha Levin =0D > Closes: https://lore.kernel.org/linux-mm/20260302151547.2389070-1-sashal@= kernel.org/=0D > Reported-by: Jiakai Xu =0D > Closes: https://lore.kernel.org/linux-mm/CAFb8wJvRhatRD-9DVmr5v5pixTMPEr3= UKjYBJjCd09OfH55CKg@mail.gmail.com/=0D > Fixes: 542eda1a8329 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas= () comments, add asserts")=0D > Signed-off-by: Lorenzo Stoakes (Oracle) =0D =0D Tested-by: Jiakai Xu =0D =0D Thanks!=0D =0D > ---=0D > mm/rmap.c | 7 +++++++=0D > 1 file changed, 7 insertions(+)=0D > =0D > diff --git a/mm/rmap.c b/mm/rmap.c=0D > index 6398d7eef393..abe4712a220c 100644=0D > --- a/mm/rmap.c=0D > +++ b/mm/rmap.c=0D > @@ -457,6 +457,13 @@ static void cleanup_partial_anon_vmas(struct vm_area= _struct *vma)=0D > list_del(&avc->same_vma);=0D > anon_vma_chain_free(avc);=0D > }=0D > +=0D > + /*=0D > + * The anon_vma assigned to this VMA is no longer valid, as we were not= =0D > + * able to correctly clone AVC state. Avoid inconsistent anon_vma tree= =0D > + * state by resetting.=0D > + */=0D > + vma->anon_vma =3D NULL;=0D > }=0D > =0D > /**=0D > --=0D > 2.53.0=