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    SubjectRe: [PATCH 5/8] mm, page_alloc: make THP-specific decisions more generic
    On Mon, 18 Jul 2016, Vlastimil Babka wrote:

    > Since THP allocations during page faults can be costly, extra decisions are
    > employed for them to avoid excessive reclaim and compaction, if the initial
    > compaction doesn't look promising. The detection has never been perfect as
    > there is no gfp flag specific to THP allocations. At this moment it checks the
    > whole combination of flags that makes up GFP_TRANSHUGE, and hopes that no other
    > users of such combination exist, or would mind being treated the same way.
    > Extra care is also taken to separate allocations from khugepaged, where latency
    > doesn't matter that much.
    >
    > It is however possible to distinguish these allocations in a simpler and more
    > reliable way. The key observation is that after the initial compaction followed
    > by the first iteration of "standard" reclaim/compaction, both __GFP_NORETRY
    > allocations and costly allocations without __GFP_REPEAT are declared as
    > failures:
    >
    > /* Do not loop if specifically requested */
    > if (gfp_mask & __GFP_NORETRY)
    > goto nopage;
    >
    > /*
    > * Do not retry costly high order allocations unless they are
    > * __GFP_REPEAT
    > */
    > if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
    > goto nopage;
    >
    > This means we can further distinguish allocations that are costly order *and*
    > additionally include the __GFP_NORETRY flag. As it happens, GFP_TRANSHUGE
    > allocations do already fall into this category. This will also allow other
    > costly allocations with similar high-order benefit vs latency considerations to
    > use this semantic. Furthermore, we can distinguish THP allocations that should
    > try a bit harder (such as from khugepageed) by removing __GFP_NORETRY, as will
    > be done in the next patch.
    >
    > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    > Acked-by: Michal Hocko <mhocko@suse.com>

    I think this is fine, but I would hope that we could check
    gfp_pfmemalloc_allowed() before compacting and failing even for costly
    orders when otherwise the first get_page_from_freelist() in the slowpath
    may have succeeded due to watermarks.

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