From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>,
Peter Feiner <pfeiner@google.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Pavel Emelyanov <xemul@parallels.com>,
Jamie Liu <jamieliu@google.com>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Andrew Morton <akpm@linux-foundation.org>,
Magnus Damm <magnus.damm@gmail.com>
Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared
Date: Tue, 26 Aug 2014 17:04:19 +0300 [thread overview]
Message-ID: <20140826140419.GA10625@node.dhcp.inet.fi> (raw)
In-Reply-To: <20140826064952.GR25918@moon>
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
next prev parent reply other threads:[~2014-08-26 14:05 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-20 21:46 [PATCH] mm: softdirty: write protect PTEs created for read faults after VM_SOFTDIRTY cleared Peter Feiner
2014-08-20 23:45 ` Kirill A. Shutemov
2014-08-21 19:37 ` Peter Feiner
2014-08-21 20:51 ` Cyrill Gorcunov
2014-08-21 21:39 ` Kirill A. Shutemov
2014-08-21 21:46 ` Peter Feiner
2014-08-21 21:51 ` Kirill A. Shutemov
2014-08-21 22:50 ` Peter Feiner
2014-08-22 6:33 ` Cyrill Gorcunov
2014-08-23 22:11 ` [PATCH v2 0/3] softdirty fix and write notification cleanup Peter Feiner
2014-08-23 22:11 ` [PATCH v2 1/3] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared Peter Feiner
2014-08-23 23:00 ` Kirill A. Shutemov
2014-08-23 23:15 ` Peter Feiner
2014-08-23 23:50 ` Kirill A. Shutemov
2014-08-24 0:55 ` Peter Feiner
2014-08-23 22:12 ` [PATCH v2 2/3] mm: mprotect: preserve special page protection bits Peter Feiner
2014-08-23 22:12 ` [PATCH v2 3/3] mm: mmap: cleanup code that preserves special vm_page_prot bits Peter Feiner
2014-08-24 1:43 ` [PATCH v3] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared Peter Feiner
2014-08-24 7:59 ` Kirill A. Shutemov
2014-08-24 19:22 ` Cyrill Gorcunov
2014-08-24 14:41 ` [PATCH v4] " Peter Feiner
2014-08-25 3:34 ` [PATCH v5] " Peter Feiner
2014-08-26 4:45 ` Hugh Dickins
2014-08-26 6:49 ` Cyrill Gorcunov
2014-08-26 14:04 ` Kirill A. Shutemov [this message]
2014-08-26 14:19 ` Cyrill Gorcunov
2014-08-26 14:56 ` Kirill A. Shutemov
2014-08-26 15:18 ` Cyrill Gorcunov
2014-08-26 15:43 ` Kirill A. Shutemov
2014-08-26 15:53 ` Cyrill Gorcunov
2014-08-27 23:12 ` Hugh Dickins
2014-08-28 6:31 ` Cyrill Gorcunov
2014-08-27 21:55 ` Hugh Dickins
2014-09-04 16:43 ` Peter Feiner
2014-09-07 21:31 ` Peter Feiner
2014-09-07 23:01 ` [PATCH v6] " Peter Feiner
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