Re: [PATCHv5 07/28] thp, mlock: do not allow huge pages in mlocked area

[Kirill A. Shutemov]

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? 

-- 
 Kirill A. Shutemov
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