Messages in this thread | | | Date | Mon, 24 Apr 2023 20:48:32 +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 8:08 PM, Michal Hocko wrote: > On Mon 24-04-23 19:40:30, Baolin Wang wrote: >> >> >> On 4/24/2023 7:34 PM, Michal Hocko wrote: >>> On Mon 24-04-23 19:20:43, Baolin Wang wrote: >>>> >>>> >>>> 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/ >>> >>> Please make it a part of the changelog. >> >> Sure. >> >>>>> >>>>>> 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. >>> >>> Does that mean that struct page is not initialized and PagePoisoned will >>> trigger or it is just zero-prefilled? >> >> In the example I provided[2], these page frames of the hole memory are >> zero-prefilled. > > OK, so make _that_ explicit in the comment. Essentially you want to say > that there are cases where we have zero-initialized struct pages for > memory holes. In general no pfn walker should touch a physical memory > range for pfn where the struct page doesn't contain any metadata it > recognizes. Zero fill struct pages do not contain any distinguishable > state so that makes it less of a problem. > > All that being said I would reformulate the comment as follows: > > * Note: the function may return non-NULL struct page even for a > * page block which contains a memory hole (i.e. there is no > * physical memory for a subset of the pfn range). This should be > * safe most of the time because struct pages are still zero > * pre-filled and pfn walkers shouldn't touch any physical memory > * range for which they do not recognize any specific metadata in > * struct pages.
Thanks. That makes sense to me. A trivial thing is I still want to add the example in the comments to make it clear. Are you okay with below description?
+ * Note: the function may return non-NULL struct page even for a page block + * which contains a memory hole (i.e. there is no physical memory for a subset + * of the pfn range). 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 should be safe most of + * the time because struct pages are still zero pre-filled and pfn walkers + * shouldn't touch any physical memory range for which they do not recognize + * any specific metadata in struct pages.
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