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