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b=j8RMXenGB2gQNPG3vT/Qlw2fo1sD9YsGDD10XoJp1SaLD1SSldoNOenju912vj/Lv y1HyOofWWmaPRUzhIwpzPSOq7Oe/qrwot5ZPEX5WCRroefzcv4ohqeg7Nv2Rs9JyfU YDYC16GL+YNCD8HSkdg3C+/++c1liSLjowwnRbRpYVW5tMx3XC5MckHsmr8vk70QjZ arXPoXFuRacQoEUnQ8zVh8N77xIpCM+PUNcC/nny5be4zJlytCRVimDK/o6ylN80wm OX+0Wn0lRplLq4PO229v6TpN3ae9ocpUg+FXi3YmMLWcGAu9IolQVHgR4bh7Rondf/ alBQie2IXPt4Q== Date: Fri, 28 Aug 2026 11:16:32 +0100 From: "Lorenzo Stoakes (ARM)" To: Kunwu Chan Cc: akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/mremap: fix locked_vm accounting for MREMAP_DONTUNMAP Message-ID: References: <20260828094823.594279-1-kunwu.chan@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260828094823.594279-1-kunwu.chan@linux.dev> X-Stat-Signature: qukiunciwoer45jzxaax9ku5xthpwwsw X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 927F580002 X-Rspam-User: X-HE-Tag: 1787912198-541335 X-HE-Meta: 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 HQcyuasE 7GqYmF/7n/uWRE1XCrRK8/Vhmxn79tVZWT82SvkXWcyZhjrSnRr3T0q3lt5M5PF1IcDYrI/+DH7HGTNUxs/0sQ5ubO7tWanWI50lWBF3MDvR8xYeuqjD2vkVzQixqPb8LT91Slcg79uHQ23a3+QowLFbuiVafnRJt8ULUe6S0SbGCI15pJDS5Vz67UAs2Si1jb2dMoI2yVCTIY2oPAeL+dLkD/Pt7Gt5M7+3JLwimmKXXw6ze+TuvpL/Zld9Nq+uR5SVlgnUe+aYUrpdCIZt/9lFTrUV1Rf/Tj7H+BTOY/KZJs9WMduryo5z7FuK/8WMQnZoZRXe/I7h5Fp+bVJ1hUbzmF2IMcrCDammdtpsXdJ9t62tRd/5CGavv+eruXb4u1+R7OzsCykhZW34mZ5nR+NJmtQr4jGkcRapCDyp2vbTmf/Lg/6NNgYhhyQhPBnKrUSZ8 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 28, 2026 at 05:48:22PM +0800, Kunwu Chan wrote: > From: Kunwu Chan > > When mremap() is called with MREMAP_DONTUNMAP on a locked VMA, > vrm_stat_account() increments mm->locked_vm for the new VMA while the > source VMA is still marked VM_LOCKED. > > For a normal mremap(), the source VMA is subsequently unmapped and the > munmap path decrements mm->locked_vm, balancing this accounting. > > With MREMAP_DONTUNMAP, the source VMA is left in place and > dontunmap_complete() clears VMA_LOCKED_MASK. When the source VMA is > subsequently unmapped, VMA_LOCKED_BIT is already clear, so the munmap > path does not undo the increment from vrm_stat_account(). This leaves > mm->locked_vm over-accounted. > > Undo the locked_vm increment in dontunmap_complete() before clearing > VMA_LOCKED_MASK. MREMAP_DONTUNMAP requires old_len == new_len, and > only VMA_LOCKED_BIT is checked to match the accounting performed by > vrm_stat_account(). Hmm I don't love this approach. > > Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()") Are you certain that my rework patch was what introduced it? > Cc: stable@vger.kernel.org > Signed-off-by: Kunwu Chan In general while the patch doesn't seem super AI-y I do see a sudden upswing in patches from you in 2026 so I do have to ask whether LLMs were used? If so please follow https://docs.kernel.org/process/coding-assistants.html (I hate having to say these things but hey, it's 2026, so.) I'm kind of inclined to do a fix myself a different way here. Given it's going to be a backported patch and you're relatively new to mm, I feel like this is the safer route, sorry about that. Happy to add a Reported-by/Closes for this however. > --- > mm/mremap.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index e8df5cdb0ac..37ea5ee1986 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -1334,6 +1334,14 @@ static void dontunmap_complete(struct vma_remap_struct *vrm, > unsigned long old_start = vrm->vma->vm_start; > unsigned long old_end = vrm->vma->vm_end; > > + /* > + * vrm_stat_account() accounted the new VMA while the source VMA > + * was still locked. Since DONTUNMAP leaves the source VMA in > + * place after clearing VMA_LOCKED_MASK, undo that accounting here. > + */ > + if (vma_test(vrm->vma, VMA_LOCKED_BIT)) > + vrm->vma->vm_mm->locked_vm -= vrm->old_len >> PAGE_SHIFT; > + I don't love this as an approach in general. As above, I think I will address this slightly differently. > /* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */ > vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK); > > @@ -1343,8 +1351,6 @@ static void dontunmap_complete(struct vma_remap_struct *vrm, > */ > if (new_vma != vrm->vma && start == old_start && end == old_end) > unlink_anon_vmas(vrm->vma); > - > - /* Because we won't unmap we don't need to touch locked_vm. */ > } > > static unsigned long move_vma(struct vma_remap_struct *vrm) > -- > 2.43.0 > -- Cheers, Lorenzo