Re: [PATCH v2 4/7] mm, page_alloc: simplify pageset_update()

[Oscar Salvador]

On Thu, Oct 08, 2020 at 01:41:58PM +0200, Vlastimil Babka wrote:
> pageset_update() attempts to update pcplist's high and batch values in a way
> that readers don't observe batch > high. It uses smp_wmb() to order the updates
> in a way to achieve this. However, without proper pairing read barriers in
> readers this guarantee doesn't hold, and there are no such barriers in
> e.g. free_unref_page_commit().
> 
> Commit 88e8ac11d2ea ("mm, page_alloc: fix core hung in free_pcppages_bulk()")
> already showed this is problematic, and solved this by ultimately only trusing
> pcp->count of the current cpu with interrupts disabled.
> 
> The update dance with unpaired write barriers thus makes no sense. Replace
> them with plain WRITE_ONCE to prevent store tearing, and document that the
> values can change asynchronously and should not be trusted for correctness.
> 
> All current readers appear to be OK after 88e8ac11d2ea. Convert them to
> READ_ONCE to prevent unnecessary read tearing, but mainly to alert anybody
> making future changes to the code that special care is needed.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Yeah, I never got my head around those smp_wmb()

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

-- 
Oscar Salvador
SUSE L3