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    SubjectRe: [PATCH rc5-mm1 1/3] mm-have-zonelist: fix memcg ooms
    On (11/03/08 21:12), Hugh Dickins didst pronounce:
    > Memcgroups up against their limit were OOMing unjustifiably on x86_32
    > with highmem: pages ripe for freeing were not being found.
    >
    > It's because the zonelist changes in vmscan.c now derive zone from gfp_mask,
    > and the gfp mask being passed into try_to_free_mem_cgroup_pages is atuned
    > to allocating a struct page_cgroup (__GFP_HIGHMEM masked off), whereas here
    > we need to be scanning highmem zones.
    >
    > Frankenstein a gfp_mask from GFP_HIGHUSER_MOVABLE and the mask passed in.
    >

    Well spotted, but I have one query below.

    > Signed-off-by: Hugh Dickins <hugh@veritas.com>
    > ---
    > Same error was in -rc3-mm1, sorry for not sending fix in time for -rc5-mm1.
    > The error comes 1 or 4 patches earlier, but this patch atop 2.6.25-rc5-mm1
    > fits best just after
    > mm-have-zonelist-contains-structs-with-both-a-zone-pointer-and-zone_idx.patch
    >
    > I've other GFP masking fixes or cleanups to come another day: this fixes
    > just one clear bug, peculiar to the -mm tree, orthogonal to the rest.
    > But two little do_try_to_free_pages cleanups follow on from this...
    >
    > mm/vmscan.c | 6 +++---
    > 1 file changed, 3 insertions(+), 3 deletions(-)
    >
    > --- 2.6.25-rc5-mm1/mm/vmscan.c 2008-03-11 14:19:36.000000000 +0000
    > +++ linux/mm/vmscan.c 2008-03-11 20:09:05.000000000 +0000
    > @@ -1444,7 +1444,6 @@ unsigned long try_to_free_mem_cgroup_pag
    > gfp_t gfp_mask)
    > {
    > struct scan_control sc = {
    > - .gfp_mask = gfp_mask,
    > .may_writepage = !laptop_mode,
    > .may_swap = 1,
    > .swap_cluster_max = SWAP_CLUSTER_MAX,
    > @@ -1454,9 +1453,10 @@ unsigned long try_to_free_mem_cgroup_pag
    > .isolate_pages = mem_cgroup_isolate_pages,
    > };
    > struct zonelist *zonelist;
    > - int target_zone = gfp_zonelist(GFP_HIGHUSER_MOVABLE);
    >
    > - zonelist = &NODE_DATA(numa_node_id())->node_zonelists[target_zone];
    > + sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
    > + (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
    > + zonelist = NODE_DATA(numa_node_id())->node_zonelists;

    While it is clear you are setting the mask to include HIGHMEM-related flags,
    it's not as clear to me why you alter the zonelist as well. target_zone was
    already based on HIGHMEM so what are you fixing there?

    It should still work as ->node_zonelists[0] is a zonelist suitable for node
    fallback as opposed to node_zonelists[1] which is for GFP_THISNODE but
    maybe this was not quite what you intended?

    Probably something obvious that will hit me the second I push send :)

    > if (do_try_to_free_pages(zonelist, sc.gfp_mask, &sc))
    > return 1;
    > return 0;
    >

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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