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Date: Tue, 26 Aug 2014 17:04:19 +0300 From: "Kirill A. Shutemov" <kirill@...temov.name> To: Cyrill Gorcunov <gorcunov@...il.com> Cc: Hugh Dickins <hughd@...gle.com>, Peter Feiner <pfeiner@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Pavel Emelyanov <xemul@...allels.com>, Jamie Liu <jamieliu@...gle.com>, Naoya Horiguchi <n-horiguchi@...jp.nec.com>, Andrew Morton <akpm@...ux-foundation.org>, Magnus Damm <magnus.damm@...il.com> Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared On Tue, Aug 26, 2014 at 10:49:52AM +0400, Cyrill Gorcunov wrote: > On Mon, Aug 25, 2014 at 09:45:34PM -0700, Hugh Dickins wrote: > > > +static int clear_refs(struct mm_struct *mm, enum clear_refs_types type, > > > + int write) > > > +{ > ... > > > + > > > + if (write) > > > + down_write(&mm->mmap_sem); > > > + else > > > + down_read(&mm->mmap_sem); > > > + > > > + if (type == CLEAR_REFS_SOFT_DIRTY) > > > + mmu_notifier_invalidate_range_start(mm, 0, -1); > > > + > > > + for (vma = mm->mmap; vma; vma = vma->vm_next) { > > > + cp.vma = vma; > > > + if (is_vm_hugetlb_page(vma)) > > > + continue; > ... > > > + if (type == CLEAR_REFS_ANON && vma->vm_file) > > > + continue; > > > + if (type == CLEAR_REFS_MAPPED && !vma->vm_file) > > > + continue; > > > + if (type == CLEAR_REFS_SOFT_DIRTY && > > > + (vma->vm_flags & VM_SOFTDIRTY)) { > > > + if (!write) { > > > + r = -EAGAIN; > > > + break; > > > > Hmm. For a long time I thought you were fixing another important bug > > with down_write, since we "always" use down_write to modify vm_flags. > > > > But now I'm realizing that if this is the _only_ place which modifies > > vm_flags with down_read, then it's "probably" safe. I've a vague > > feeling that this was discussed before - is that so, Cyrill? > > Well, as far as I remember we were not talking before about vm_flags > and read-lock in this function, maybe it was on some unrelated lkml thread > without me CC'ed? Until I miss something obvious using read-lock here > for vm_flags modification should be safe, since the only thing which is > important (in context of vma-softdirty) is the vma's presence. Hugh, > mind to refresh my memory, how long ago the discussion took place? It seems safe in vma-softdirty context. But if somebody else will decide that it's fine to modify vm_flags without down_write (in their context), we will get trouble. Sasha will come with weird bug report one day ;) At least vm_flags must be updated atomically to avoid race in middle of load-modify-store. -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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