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    SubjectRe: [PATCH v3] mm, compaction: Skip all non-migratable pages during scan
    Date
    David Hildenbrand <david@redhat.com> writes:

    > On 17.05.23 18:15, Khalid Aziz wrote:
    >> Pages pinned in memory through extra refcounts can not be migrated.
    >> Currently as isolate_migratepages_block() scans pages for
    >> compaction, it skips any pinned anonymous pages. All non-migratable
    >> pages should be skipped and not just the anonymous pinned pages.
    >> This patch adds a check for extra refcounts on a page to determine
    >> if the page can be migrated. This was seen as a real issue on a
    >> customer workload where a large number of pages were pinned by vfio
    >> on the host and any attempts to allocate hugepages resulted in
    >> significant amount of cpu time spent in either direct compaction or
    >> in kcompactd scanning vfio pinned pages over and over again that can
    >> not be migrated.
    >
    > How will this change affect alloc_contig_range(), such as used for CMA
    > allocations or virtio-mem? alloc_contig_range() ends up calling
    > isolate_migratepages_range() -> isolate_migratepages_block().

    IIUC, cc->alloc_contig can be used to distinguish contiguous allocation
    and compaction. And, from the original commit which introduced
    anonymous pages skipping (commit 119d6d59dcc0 ("mm, compaction: avoid
    isolating pinned pages ")) and this patch, large number of migration
    failure during compaction causes real issue too. So, I suggest to use
    cc->alloc_contig here.

    > We don't want to fail early in case there is a short-term pin that
    > might go away any moment after we isolated ... that will make the
    > situation worse for these use cases, especially if MIGRATE_CMA or
    > ZONE_MOVABLE is involved.

    Best Regards,
    Huang, Ying

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