Re: [PATCH 1/9] mm/page_alloc: don't undo a racing start_isolate_page_range()

[Gregory Price (Meta)]

On 2026-10-27 16:46 +0100, Vlastimil Babka (SUSE) wrote:
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
> start_isolate_page_range() fails with -EBUSY if one of the pageblocks is
> already isolated, i.e. if somebody else is working on it. It restores
> the pageblocks which it has isolated itself before it returns, as its
> kernel-doc says:
> 
>   We then clean up by restoring the migration type on pageblocks we may
>   have modified and return -EBUSY to caller. This prevents two threads
>   from simultaneously working on overlapping ranges.
> 
> alloc_contig_frozen_range() nevertheless calls undo_isolate_page_range()
> for the whole range in this case, which clears every isolated pageblock
> it finds. The only ones left at this point belong to the other user of
> the range:
> 
>   CPU0 offline_pages()                CPU1 alloc_contig_frozen_range()
>   start_isolate_page_range(A)
>     pageblock X: isolated
>   migrate ...
>                                       start_isolate_page_range(B)
>                                         X is isolated: -EBUSY
>                                         (own pageblocks restored)
>                                       undo_isolate_page_range(B)
>                                         X: not isolated any more
>   test_pages_isolated(A): -EBUSY
>   (again and again)
> 
> offline_pages() repeats the migration and test_pages_isolated() until the
> latter succeeds, which is now impossible and the task spins until it gets
> a signal, while the free pages of X are handed out by the allocator again.
> 
> If X is released after the last test_pages_isolated(),
> __offline_isolated_pages() takes pages which are accounted as free off
> the free lists as MIGRATE_ISOLATE. Thus NR_FREE_PAGES stays too high, or
> hits BUG_ON(!PageBuddy(page)) if one of them has been allocated.
> 
> For another alloc_contig_frozen_range() or cma_alloc() the result is a
> spurious -EBUSY.
> 
> Return the error, like the function did until it was reworked.
> 
> Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Given i separately found this exact patch and tested it

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
Tested-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>

-- 
~Gregory