Messages in this thread | | | Date | Mon, 5 Sep 2022 10:38:51 +0200 | Subject | Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast | From | David Hildenbrand <> |
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On 04.09.22 18:52, Aneesh Kumar K V wrote: > On 9/2/22 12:02 PM, David Hildenbrand wrote: >> On 01.09.22 20:35, Yang Shi wrote: >>> On Thu, Sep 1, 2022 at 11:07 AM Peter Xu <peterx@redhat.com> wrote: >>>> >>>> On Thu, Sep 01, 2022 at 10:50:48AM -0700, Yang Shi wrote: >>>>> Yeah, because THP collapse does copy the data before clearing pte. If >>>>> we want to remove pmdp_collapse_flush() by just clearing pmd, we >>>>> should clear *AND* flush pte before copying the data IIRC. >>>> >>>> Yes tlb flush is still needed. IIUC the generic pmdp_collapse_flush() will >>>> still be working (with the pte level flushing there) but it should just >>>> start to work for all archs, so potentially we could drop the arch-specific >>>> pmdp_collapse_flush()s, mostly the ppc impl. >>> >>> I'm don't know why powperpc needs to have its specific >>> pmdp_collapse_flush() in the first place, not only the mandatory IPI >>> broadcast, but also the specific implementation of pmd tlb flush. But >>> anyway the IPI broadcast could be removed at least IMO. >>> >> >> pmdp_collapse_flush() is overwritten on book3s only. It either translates >> to radix__pmdp_collapse_flush() or hash__pmdp_collapse_flush(). >> >> >> radix__pmdp_collapse_flush() has a comment explaining the situation: >> >> >> + /* >> + * pmdp collapse_flush need to ensure that there are no parallel gup >> + * walk after this call. This is needed so that we can have stable >> + * page ref count when collapsing a page. We don't allow a collapse page >> + * if we have gup taken on the page. We can ensure that by sending IPI >> + * because gup walk happens with IRQ disabled. >> + */ >> >> >> The comment for hash__pmdp_collapse_flush() is a bit more involved: >> >> /* >> * Wait for all pending hash_page to finish. This is needed >> * in case of subpage collapse. When we collapse normal pages >> * to hugepage, we first clear the pmd, then invalidate all >> * the PTE entries. The assumption here is that any low level >> * page fault will see a none pmd and take the slow path that >> * will wait on mmap_lock. But we could very well be in a >> * hash_page with local ptep pointer value. Such a hash page >> * can result in adding new HPTE entries for normal subpages. >> * That means we could be modifying the page content as we >> * copy them to a huge page. So wait for parallel hash_page >> * to finish before invalidating HPTE entries. We can do this >> * by sending an IPI to all the cpus and executing a dummy >> * function there. >> */ >> >> I'm not sure if that implies that the IPI is needed for some other hash-magic. >> >> Maybe Aneesh can clarify. >> > > We still need the IPI for the hash. Another reason for architecture to override that > function is to help them use the right page size when flushing the TLB.
Thanks for clarifying. So the radix variant wouldn't need the IPI anymore, once GUP-fast is handled differently, correct?
-- Thanks,
David / dhildenb
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