Messages in this thread | | | Date | Sun, 23 Apr 2023 14:00:05 +0800 | Subject | Re: [PATCH 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() | From | Baolin Wang <> |
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On 4/23/2023 1:19 PM, Mike Rapoport wrote: > Hi, > > On Sat, Apr 22, 2023 at 06:15:18PM +0800, Baolin Wang wrote: >> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which >> checks whether the given zone contains holes, and uses pfn_to_online_page() >> to validate if the start pfn is online and valid, as well as using pfn_valid() >> to validate the end pfn. >> >> However, though the start pfn of a pageblock is valid, it can not always >> guarantee the end pfn of the pageblock is also valid (may be holes) in some >> cases. For example, if the pageblock order is MAX_ORDER - 1, which will fall > > Nit: in the current mm tree the default pageblock order is MAX_ORDER.
Ah, yes, will change in next version.
> >> into 2 sub-sections, and the end pfn of the pageblock may be hole even though >> the start pfn is online and valid. >> >> This did not break anything until now, but the zone continuous is fragile >> in this possible scenario. So as previous discussion[1], it is better to >> add some comments to explain this possible issue in case there are some >> future pfn walkers that rely on this. >> >> [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@yhuang6-desk2.ccr.corp.intel.com/ >> >> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> >> --- >> mm/page_alloc.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..dc4005b32ae0 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,14 @@ void __free_pages_core(struct page *page, unsigned int order) >> * interleaving within a single pageblock. It is therefore sufficient to check >> * the first and last page of a pageblock and avoid checking each individual >> * page in a pageblock. >> + * >> + * Note: if the start pfn of a pageblock is valid, but it can not always guarantee >> + * the end pfn of the pageblock is also valid (may be holes) in some cases. For >> + * example, if the pageblock order is MAX_ORDER - 1, which will fall into 2 >> + * sub-sections, and the end pfn of the pageblock may be hole even though the >> + * start pfn is online and valid. This did not break anything until now, but be >> + * careful this possible issue when checking if the whole pfns are valid of a > > careful about ...
OK. Thanks for reviewing.
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