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Date: Fri, 14 Feb 2020 03:36:40 +0300 From: "Kirill A. Shutemov" <kirill@...temov.name> To: Brian Geffon <bgeffon@...gle.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, "Michael S . Tsirkin" <mst@...hat.com>, Arnd Bergmann <arnd@...db.de>, LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, Linux API <linux-api@...r.kernel.org>, Andy Lutomirski <luto@...capital.net>, Will Deacon <will@...nel.org>, Andrea Arcangeli <aarcange@...hat.com>, Sonny Rao <sonnyrao@...gle.com>, Minchan Kim <minchan@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, Yu Zhao <yuzhao@...gle.com>, Jesse Barnes <jsbarnes@...gle.com>, Nathan Chancellor <natechancellor@...il.com>, Florian Weimer <fweimer@...hat.com> Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap(). On Thu, Feb 13, 2020 at 10:20:44AM -0800, Brian Geffon wrote: > Hi Kirill, > > > But if you do the operation for the VM_LOCKED vma, you'll have two locked > > VMA's now, right? Where do you account the old locked vma you left behind? > > You bring up a good point. In a previous iteration of my patch I had > it clearing the locked flags on the old VMA as technically the locked > pages had migrated. I talked myself out of that but the more I think > about it we should probably do that. Something along the lines of: > > + if (vm_flags & VM_LOCKED) { > + /* Locked pages would have migrated to the new VMA */ > + vma->vm_flags &= VM_LOCKED_CLEAR_MASK; > + if (new_len > old_len) > + mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT; > + } > > I feel that this is correct. The only other possible option would be > to clear only the VM_LOCKED flag on the old vma leaving VM_LOCKONFAULT > to handle the MCL_ONFAULT mlocked situation, thoughts? Regardless I'll > have to mail a new patch because that part where I'm incrementing the > mm->locked_vm lost the check on VM_LOCKED during patch versions. Note, that we account mlock limit on per-VMA basis, not per page. Even for VM_LOCKONFAULT. > Thanks again for taking the time to review. I believe the right approach is to strip VM_LOCKED[ONFAULT] from the vma you left behind. Or the new vma. It is a policy decision. JFYI, we do not inherit VM_LOCKED on fork(), so it's common practice to strip VM_LOCKED on vma duplication. Other option is to leave VM_LOCKED on both VMAs and fail the operation if we are over the limit. But we need to have a good reason to take this path. It makes the interface less flexible. -- Kirill A. Shutemov
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