On Tue, Aug 26, 2014 at 06:19:14PM +0400, Cyrill Gorcunov wrote: > On Tue, Aug 26, 2014 at 05:04:19PM +0300, Kirill A. Shutemov wrote: > > > > > > > > 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. > > Which race you mean here? Two concurrent clear-refs? Two concurent clear-refs is fine. But if somebody else will exploit the same approch to set/clear other VM_FOO and it will race with clear-refs we get trouble: some modifications can be lost. Basically, it's safe if only soft-dirty is allowed to modify vm_flags without down_write(). But why is soft-dirty so special? Should we consider moving protection of some vma fields under per-vma lock rather use over-loaded mmap_sem? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>