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mm: compaction: use the proper flag to determine watermarks

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There are 4 NUMA nodes on my machine, and each NUMA node has 32GB of
memory.  I have configured 16GB of CMA memory on each NUMA node, and
starting a 32GB virtual machine with device passthrough is extremely slow,
taking almost an hour.

Long term GUP cannot allocate memory from CMA area, so a maximum of 16 GB
of no-CMA memory on a NUMA node can be used as virtual machine memory. 
There is 16GB of free CMA memory on a NUMA node, which is sufficient to
pass the order-0 watermark check, causing the __compaction_suitable()
function to consistently return true.

For costly allocations, if the __compaction_suitable() function always
returns true, it causes the __alloc_pages_slowpath() function to fail to
exit at the appropriate point.  This prevents timely fallback to
allocating memory on other nodes, ultimately resulting in excessively long
virtual machine startup times.

Call trace:
__alloc_pages_slowpath
    if (compact_result == COMPACT_SKIPPED ||
        compact_result == COMPACT_DEFERRED)
        goto nopage; // should exit __alloc_pages_slowpath() from here

We could use the real unmovable allocation context to have
__zone_watermark_unusable_free() subtract CMA pages, and thus we won't
pass the order-0 check anymore once the non-CMA part is exhausted.  There
is some risk that in some different scenario the compaction could in fact
migrate pages from the exhausted non-CMA part of the zone to the CMA part
and succeed, and we'll skip it instead.  But only __GFP_NORETRY
allocations should be affected in the immediate "goto nopage" when
compaction is skipped, others will attempt with DEF_COMPACT_PRIORITY
anyway and won't fail without trying to compact-migrate the non-CMA
pageblocks into CMA pageblocks first, so it should be fine.

After this fix, it only takes a few tens of seconds to start a 32GB
virtual machine with device passthrough functionality.

Link: https://lore.kernel.org/lkml/1736335854-548-1-git-send-email-yangge1116@126.com/
Link: https://lkml.kernel.org/r/1737788037-8439-1-git-send-email-yangge1116@126.com
Signed-off-by: yangge <yangge1116@126.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Barry Song <21cnbao@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
yangge 2025-01-25 14:53:57 +08:00 • committed by Andrew Morton
parent 64c37e134b
commit 6268f0a166
1 changed files with 25 additions and 4 deletions
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29
mm/compaction.c
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@ -2491,7 +2491,8 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
*/
static enum compact_result
compaction_suit_allocation_order(struct zone *zone, unsigned int order,
int highest_zoneidx, unsigned int alloc_flags)
int highest_zoneidx, unsigned int alloc_flags,
bool async)
{
unsigned long watermark;
@ -2500,6 +2501,23 @@ compaction_suit_allocation_order(struct zone *zone, unsigned int order,
alloc_flags))
return COMPACT_SUCCESS;
/*
* For unmovable allocations (without ALLOC_CMA), check if there is enough
* free memory in the non-CMA pageblocks. Otherwise compaction could form
* the high-order page in CMA pageblocks, which would not help the
* allocation to succeed. However, limit the check to costly order async
* compaction (such as opportunistic THP attempts) because there is the
* possibility that compaction would migrate pages from non-CMA to CMA
* pageblock.
*/
if (order > PAGE_ALLOC_COSTLY_ORDER && async &&
!(alloc_flags & ALLOC_CMA)) {
watermark = low_wmark_pages(zone) + compact_gap(order);
if (!__zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
0, zone_page_state(zone, NR_FREE_PAGES)))
return COMPACT_SKIPPED;
}
if (!compaction_suitable(zone, order, highest_zoneidx))
return COMPACT_SKIPPED;
@ -2535,7 +2553,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
if (!is_via_compact_memory(cc->order)) {
ret = compaction_suit_allocation_order(cc->zone, cc->order,
cc->highest_zoneidx,
cc->alloc_flags);
cc->alloc_flags,
cc->mode == MIGRATE_ASYNC);
if (ret != COMPACT_CONTINUE)
return ret;
}
@ -3038,7 +3057,8 @@ static bool kcompactd_node_suitable(pg_data_t *pgdat)
ret = compaction_suit_allocation_order(zone,
pgdat->kcompactd_max_order,
highest_zoneidx, ALLOC_WMARK_MIN);
highest_zoneidx, ALLOC_WMARK_MIN,
false);
if (ret == COMPACT_CONTINUE)
return true;
}
@ -3079,7 +3099,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
continue;
ret = compaction_suit_allocation_order(zone,
cc.order, zoneid, ALLOC_WMARK_MIN);
cc.order, zoneid, ALLOC_WMARK_MIN,
false);
if (ret != COMPACT_CONTINUE)
continue;