On Tue, May 19, 2015 at 04:37:25PM +0200, Vlastimil Babka wrote: > On 05/15/2015 03:41 PM, Kirill A. Shutemov wrote: > >On Fri, May 15, 2015 at 02:56:42PM +0200, Vlastimil Babka wrote: > >>On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote: > >>>With new refcounting THP can belong to several VMAs. This makes tricky > >>>to track THP pages, when they partially mlocked. It can lead to leaking > >>>mlocked pages to non-VM_LOCKED vmas and other problems. > >>>With this patch we will split all pages on mlock and avoid > >>>fault-in/collapse new THP in VM_LOCKED vmas. > >>> > >>>I've tried alternative approach: do not mark THP pages mlocked and keep > >>>them on normal LRUs. This way vmscan could try to split huge pages on > >>>memory pressure and free up subpages which doesn't belong to VM_LOCKED > >>>vmas. But this is user-visible change: we screw up Mlocked accouting > >>>reported in meminfo, so I had to leave this approach aside. > >>> > >>>We can bring something better later, but this should be good enough for > >>>now. > >> > >>I can imagine people won't be happy about losing benefits of THP's when they > >>mlock(). > >>How difficult would it be to support mlocked THP pages without splitting > >>until something actually tries to do a partial (un)mapping, and only then do > >>the split? That will support the most common case, no? > > > >Yes, it will. > > > >But what will we do if we fail to split huge page on munmap()? Fail > >munmap() with -EBUSY? > > We could just unmlock the whole THP page and if we could make the deferred > split done ASAP, and not waiting for memory pressure, the window with > NR_MLOCK being undercounted would be minimized. Since the RLIMIT_MEMLOCK is > tracked independently from NR_MLOCK, there should be no danger wrt breaching > the limit due to undercounting here? I'm not sure what "ASAP" should mean here and how to implement it. I would really prefer to address mlock separately. The patchset is already huge enough. :-/ -- 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>