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b=O23RZQMjulYOhifaRc1NeJG7nl0tKpXfmEe0BIXglKp1iA2azgR4hFzMd/8zWZUtc oPmr7wQ3SgJT8kM74j882Agi7wbjlOoJ1E9fO6/QNk7o1C1yQrYpIsi49tPUgb98Ve WoGD/KW7X6blZ7m2IdXnMLIdiaBLJ8lYFKKiYg4HcVKAQVdkvvX6aE7aZOJsQ/zxu9 RnJHoX2cCPZnlxV9ceg0zux8dDibrLDr8lDQavSb/3eLIYghxLB50SxXXESePfoJto 8yMSa5y50g0cm1NTZVal5sZxJhVoiGC6vd1MS7rdpWGIb/g1/UzkhwPjiBpfiJW5At FzUunG6xRhrog== Message-ID: Date: Tue, 9 Jun 2026 16:16:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 02/12] mm/rmap: Add try_to_unmap_hugetlb_one To: Dev Jain , akpm@linux-foundation.org, ljs@kernel.org, chrisl@kernel.org, kasong@tencent.com, hughd@google.com, liam@infradead.org Cc: riel@surriel.com, vbabka@kernel.org, harry@kernel.org, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, youngjun.park@lge.com, baolin.wang@linux.alibaba.com, pfalcato@suse.de, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20260526063635.61721-1-dev.jain@arm.com> <20260526063635.61721-3-dev.jain@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 94930180018 X-Stat-Signature: 8z1gfdtimte1xoyaykti878qnp7cti1y X-Rspam-User: X-HE-Tag: 1781014617-311382 X-HE-Meta: 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 ApuOBLex e5oJBR4JUkfzESoRZP1v4vvMZoBrgiCMIYK3//D9/ME4e4iHmci0m4CUMX30tYKJ2cl4WUVQIgheNkCCIWWjjYfnqdqGs5lBGKHXuunDlmjhrFwf7ZrAGoNTCtLF4q1mL4n6Uf6AlKzs8nCIC6xzCGiBovubpZaGT7j7aGf86EXUJ3SkP0hSDwdLwjJHBiLV1x3RfKPkZYWiFMklYXosSNE4keoNBnYQQ1Ikmqzg1Zvj4IokTbl4IjjrqUXF7oeENKZLr8Tnj6pKyTT0Nty22c1prZuyb2O8TEFFmHbowYWuxuUWIlobPOElswC/hC1VSkkEOZnDd/P3Pa4boF/HsvJzbJA1ucfmep/bqfxC8U+TS+tI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/26/26 08:36, Dev Jain wrote: > Simplify try_to_unmap_one by separating out the hugetlb parts into > try_to_unmap_hugetlb_one. > > To understand the correctness of the refactoring, the following points > are noted: > > 1. try_to_unmap() is called for hugetlb folios only when they are > hwpoisoned. Ack > > 2. A hugetlb VMA cannot be mlocked. Ack > > 3. The pvmw API sets pvmw.pte to the base of the hugetlb folio (pvmw.pmd > is NULL). Ack > > 4. We won't ever process a softleaf entry that encodes a hugetlb folio; > hugetlb folios are never swapped out, migration entries will be skipped > (PVMW_MIGRATION not passed) and device-exclusive does not work for > hugetlb. Ack, we should always find present entries. > > 5. uffd-wp bit is lost when converting pvmw.pte to hwpoison softleaf > (therefore no need to call pte_install_uffd_wp_if_needed after > clearing pvmw.pte) Are you sure? And if so, can you elaborate why + document? > > 6. TTU_HWPOISON is always present; for it to not be present, either folio > has to be in swapcache, or mapping_can_writeback() is true (see > unmap_poisoned_folio), none of which is true for hugetlb folios. Agreed. That's one very confusing bit in the existing code. > > 7. hugetlb uses separate counters from normal rss counters, therefore > update_highwater_rss() need not be called. Ack. > > While at it: > > - Change VM_BUG_* to VM_WARN_*. > > - Do not declare variables which are only used once > > - Assert that the subpage derived by the pvmw walk is exactly the head > page. This is because try_to_unmap() does not remember the specific > subpage which was hwpoisoned, and, since we cannot munmap/mremap > across a hugetlb folio partially, the first pte mapping the hugetlb > folio (in case of a contpte or contpmd mapped folio) cannot ever point > to an intermediate page. Might want to add a Suggested-by:, so people know directly whom to blame :) I am very much in favor of decoupling both things cleanly, instead of hacking it together and make the code all big and confusing. > > Signed-off-by: Dev Jain > --- > mm/rmap.c | 203 ++++++++++++++++++++++++++++++++---------------------- > 1 file changed, 121 insertions(+), 82 deletions(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 430c91c8fe2ae..06ab1158d4cd1 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1978,6 +1978,121 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, > FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY); > } > > +static bool __try_to_unmap_hugetlb_one(struct folio *folio, > + struct vm_area_struct *vma, struct page_vma_mapped_walk *pvmw, > + struct mmu_notifier_range *range, enum ttu_flags flags) > +{ > + unsigned long hsz = huge_page_size(hstate_vma(vma)); Can be const. > + unsigned long address = pvmw->address; Can be const. But do we even need the temporary variable? > + struct mm_struct *mm = vma->vm_mm; > + struct page *subpage; a) subpages do not exist. It's just a folio page. b) subpages are irrelevant for hugetlb, just drop anything that deals with them. > + bool ret = true; > + pte_t pteval; > + Worth adding a comment here, why we don't need a loop. /* There is only a single mapping in a VMA. */ > + if (!page_vma_mapped_walk(pvmw)) > + return true; > + > + pteval = ptep_get(pvmw->pte); Should we now properly use huge_ptep_get()? > + VM_WARN_ON(!pte_present(pteval)); > + subpage = folio_page(folio, pte_pfn(pteval) - folio_pfn(folio)); No need for subpages. > + VM_WARN_ON(folio_page(folio, 0) != subpage); VM_WARN_ON(pte_pfn(pteval) != folio_pfn(folio)); I assume both can be VM_WARN_ON_ONCE() > + > + /* > + * huge_pmd_unshare may unmap an entire PMD page. There is no way of > + * knowing exactly which PMDs may be cached for this mm, so we must > + * flush them all. start/end were already adjusted above to cover this > + * range. > + */ > + flush_cache_range(vma, range->start, range->end); > + > + /* > + * To call huge_pmd_unshare, i_mmap_rwsem must be held in write mode. > + * Caller needs to explicitly do this outside rmap routines. > + * > + * We also must hold hugetlb vma_lock in write mode. Lock order dictates > + * acquiring vma_lock BEFORE i_mmap_rwsem. We can only try lock here and > + * fail if unsuccessful. > + */ > + if (!folio_test_anon(folio)) { > + struct mmu_gather tlb; > + > + VM_WARN_ON(!(flags & TTU_RMAP_LOCKED)); > + if (!hugetlb_vma_trylock_write(vma)) { > + ret = false; > + goto walk_done; > + } > + > + tlb_gather_mmu_vma(&tlb, vma); > + if (huge_pmd_unshare(&tlb, vma, address, pvmw->pte)) { > + hugetlb_vma_unlock_write(vma); > + huge_pmd_unshare_flush(&tlb, vma); > + tlb_finish_mmu(&tlb); > + /* > + * The PMD table was unmapped, consequently unmapping > + * the folio. > + */ > + goto walk_done; > + } > + hugetlb_vma_unlock_write(vma); > + tlb_finish_mmu(&tlb); > + } > + pteval = huge_ptep_clear_flush(vma, address, pvmw->pte); > + if (pte_dirty(pteval)) Should we now use huge_pte_dirty() for consistency? > + folio_mark_dirty(folio); > + > + VM_WARN_ON(!(flags & TTU_HWPOISON)); This should be checked right at the very beginning of try_to_unmap_hugetlb_one(). > + pteval = swp_entry_to_pte(make_hwpoison_entry(subpage)); > + hugetlb_count_sub(folio_nr_pages(folio), mm); > + set_huge_pte_at(mm, address, pvmw->pte, pteval, hsz); > + hugetlb_remove_rmap(folio); > + folio_put_refs(folio, 1); > + > +walk_done: > + page_vma_mapped_walk_done(pvmw); > + return ret; > +} > + > +static bool try_to_unmap_hugetlb_one(struct folio *folio, > + struct vm_area_struct *vma, unsigned long address, void *arg) > +{ > + DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0); > + struct mmu_notifier_range range; > + enum ttu_flags flags = (enum ttu_flags)(long)arg; > + bool ret; > + > + /* > + * The try_to_unmap() is only passed a hugetlb folio in the case > + * where the hugetlb folio contains a poisoned page. > + */ > + VM_WARN_ON_FOLIO(!folio_contain_hwpoisoned_page(folio), folio); As discussed, I'd prefer to just have folio_test_hwpoison() here. > + > + /* > + * When racing against e.g. zap_pte_range() on another cpu, > + * in between its ptep_get_and_clear_full() and folio_remove_rmap_*(), > + * try_to_unmap() may return before folio_mapped() has become false, > + * if page table locking is skipped: use TTU_SYNC to wait for that. > + */ The comment should be updated to mention hugetlb primitives ... but ... it's a good question whether that is required *at all* ... > + if (flags & TTU_SYNC) > + pvmw.flags = PVMW_SYNC; ... and in fact, looking at page_vma_mapped_walk(), PVMW_SYNC is irrelevant. > + > + /* > + * For hugetlb, it could be much worse than THP if we need pud > + * invalidation in the case of pmd sharing. I don't understand that comment. Is it really helpful? > + * > + * Note that the folio can not be freed in this function as call of "cannot" ? > + * try_to_unmap() must hold a reference on the folio. > + */ Do we really need this comment entirely? We don't even look at the folio after unmapping it. > + range.end = vma_address_end(&pvmw); > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, > + address, range.end); > + adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); > + mmu_notifier_invalidate_range_start(&range); > + ret = __try_to_unmap_hugetlb_one(folio, vma, &pvmw, &range, > + flags); Is it really worth it moving that stuff to a separate function? > + mmu_notifier_invalidate_range_end(&range); > + return ret; > +} > + -- Cheers, David