Messages in this thread | | | Date | Tue, 19 May 2015 16:37:25 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCHv5 07/28] thp, mlock: do not allow huge pages in mlocked area |
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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?
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