Re: [PATCHv2] mm: check high-order page when free it to pcp

[Zhaoyang Huang]

On Tue, Sep 20, 2022 at 4:55 PM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 20, 2022 at 08:49:00AM +0800, zhaoyang.huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > High-order pcp page escaped from checking when both of DEBUG_VM and debug_pagealloc
> > are all disabled if it allocated again from pcp_list without going to global
> > free list, which should be introduced by 44042b4498.
> >
> > fix:44042b4498
> >
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> Why is the order check needed? In this case, I think the comment itself was
> wrong and should be corrected. free_pcp_prepare is called when dealing with
> a PCP page being freed and these days that may order-0 or high-order pages.
> Wieh DEBUG_VM disabled, the pages are checked in by bulkfree_pcp_prepare
> when freeing pages from PCP to the core allocator. Why would just
> high-order pages be checked in free_pcp_prepare and checked again in
> bulkfree_pcp_prepare?
for high-order pcp pages(head page in fact), they might escape from
bulkfree_pcp_prepare if they were allocated from pcp-list
again(pcp->count < high) prior to going to core allocator.
for order-0 pcp pages, they will not be checked at all for the purpose
of reducing overhead.
>
> --
> Mel Gorman
> SUSE Labs