Messages in this thread | | | Date | Mon, 24 Apr 2023 19:20:43 +0800 | Subject | Re: [PATCH v2 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 4/24/2023 5:54 PM, Michal Hocko wrote: > On Sun 23-04-23 18:59:11, 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, the __pageblock_pfn_to_page() function may return non-NULL even >> if the end pfn of a pageblock is in a memory hole in some situations. For >> example, if the pageblock order is MAX_ORDER, 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 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/ > > Do I remember correctly you've had a specific configuration that would > trigger this case?
Yes, I provided an example in previous thread [2] so show the __pageblock_pfn_to_page() is fragile in some cases.
[2] https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@linux.alibaba.com/
> >> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> >> --- >> Changes from v1: >> - Update the comments per Ying and Mike, thanks. >> --- >> mm/page_alloc.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..9756d66f471c 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,13 @@ 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: the function may return non-NULL even if the end pfn of a pageblock >> + * is in a memory hole in some situations. For example, if the pageblock >> + * order is MAX_ORDER, 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 about this possible >> + * issue when checking whether all pfns of a pageblock are valid. > > It is not really clear what you should be doing (other than to be > careful which is not helpful much TBH) when you encounter this > situation. If the reality changes and this would break in the future > what would breakage look like? What should be done about that?
That depends on what the future pfn walkers do, which may access some hole memory with zero-init page frame. For example, if checking the __PageMovable() for a zero-init page frame, that will crash the system. But I can not list all the possible cases.
So how about below words?
* Note: the function may return non-NULL even if the end pfn of a pageblock * is in a memory hole in some situations. For example, if the pageblock * order is MAX_ORDER, 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 about this possible * issue when checking whether all pfns of a pageblock are valid, that may * lead to accessing empty page frame, and the worst case can crash the system. * So you should use pfn_to_onlie_page() instead of pfn_valid() to valid the * pfns in a pageblock if such case happens.
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