lkml.org 
[lkml]   [2018]   [Oct]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask
    From
    Date
    On 10/19/18 10:06 AM, Michal Hocko wrote:
    > On Thu 18-10-18 19:11:47, Andrew Morton wrote:
    >> On Wed, 26 Sep 2018 16:22:27 +0200 Michal Hocko <mhocko@kernel.org> wrote:
    >>
    >>>> MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask.
    >>>> __GFP_THISNODE is applied only when we are not using
    >>>> __GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask
    >>>> now.
    >>>> Lastly MPOL_BIND wasn't handled explicitly but in the end the removed
    >>>> late check would remove __GFP_THISNODE for it as well. So in the end we
    >>>> are doing the same thing unless I miss something
    >>>
    >>> Forgot to add. One notable exception would be that the previous code
    >>> would allow to hit
    >>> WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
    >>> in policy_node if the requested node (e.g. cpu local one) was outside of
    >>> the mbind nodemask. This is not possible now. We haven't heard about any
    >>> such warning yet so it is unlikely that it happens though.
    >>
    >> Perhaps a changelog addition is needed to cover the above?
    >
    > : THP allocation mode is quite complex and it depends on the defrag
    > : mode. This complexity is hidden in alloc_hugepage_direct_gfpmask from a
    > : large part currently. The NUMA special casing (namely __GFP_THISNODE) is
    > : however independent and placed in alloc_pages_vma currently. This both
    > : adds an unnecessary branch to all vma based page allocation requests and
    > : it makes the code more complex unnecessarily as well. Not to mention
    > : that e.g. shmem THP used to do the node reclaiming unconditionally
    > : regardless of the defrag mode until recently. This was not only
    > : unexpected behavior but it was also hardly a good default behavior and I
    > : strongly suspect it was just a side effect of the code sharing more than
    > : a deliberate decision which suggests that such a layering is wrong.
    > :
    > : Moreover the oriinal code allowed to trigger
    > : WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
    > : in policy_node if the requested node (e.g. cpu local one) was outside of
    > : the mbind nodemask. This is not possible now. We haven't heard about any
    > : such warning yet so it is unlikely that it happens but still a signal of
    > : a wrong code layering.

    Ah, as I said in the other mail, I think it's inaccurate, the warning
    was not possible to hit.

    There's also a slight difference wrt MPOL_BIND. The previous code would
    avoid using __GFP_THISNODE if the local node was outside of
    policy_nodemask(). After your patch __GFP_THISNODE is avoided for all
    MPOL_BIND policies. So there's a difference that if local node is
    actually allowed by the bind policy's nodemask, previously
    __GFP_THISNODE would be added, but now it won't be. I don't think it
    matters that much though, but maybe the changelog could say that
    (instead of the inaccurate note about warning). Note the other policy
    where nodemask is relevant is MPOL_INTERLEAVE, and that's unchanged by
    this patch.

    When that's addressed, you can add

    Acked-by: Vlastimil Babka <vbabka@suse.cz>

    (Note I also agree with patch 1/2 but didn't think it was useful to
    formally ack it on top of Mel's ack supported by actual measurements, as
    we're all from the same company).

    > : Get rid of the thp special casing from alloc_pages_vma and move the logic
    > : to alloc_hugepage_direct_gfpmask. __GFP_THISNODE is applied to
    > : the resulting gfp mask only when the direct reclaim is not requested and
    > : when there is no explicit numa binding to preserve the current logic.
    > :
    > : This allows for removing alloc_hugepage_vma as well.
    >
    > Better?
    >
    >> I assume that David's mbind() concern has gone away.
    >
    > Either I've misunderstood it or it was not really a real issue.
    >

    \
     
     \ /
      Last update: 2018-10-22 15:29    [W:3.799 / U:0.128 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site