mm: prevent MIGRATE_ISOLATE pages entering other free lists
It is observed that, during a CMA allocation,
test_pages_isolated() fails sometimes.
In low memory situations, this results in continuous
failures of CMA allocations.
When testing for isolation, test_pages_isolated finds
that some pages are allocated, which were available on
pcp list when start_isolate_page_range marked the
pageblock as MIGRATE_ISOLATE. These pages, though
marked as isolated in the respective pageblock, are
moved to non-isolate freelists while being drained.
This results in these pages being allocated from other
paths while a contig allocation is in progress. In
certain cases, these pages get allocated from the
migrate_pages() of current alloc_contig_range() itself.
In the function free_pcppages_bulk(), when a page is
moved to buddy, the migrate list is picked based on
page_private. In the case of CMA pages, which were on
pcp list during start_isolate_page_range, page_private
will point to MIGRATE_CMA, and the pageblock migrate
type can be MIGRATE_ISOLATE. This results in these pages
being moved to cma freelist, rather than isolate freelist,
inside free_pcppages_bulk. This means they can get allocated.
If cma pages are never allowed to enter pcp lists, we
will not hit this issue. This is partly done by
free_hot_cold_page(), where we prevent cma pages
entering pcp. But cma pages can still get on pcp lists
through rmqueue_bulk. Preventing cma pages enter pcp
via rmqueue_bulk, can result in underutilization of
CMA pages for non-contiguous allocations, as rmqueue_bulk
is the frequent path for order 0 allocations.
CRs-fixed:720761
Change-Id: I01b12e6455c75519f1dce5d31467123cb1eb54b7
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
1 file changed