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Wed, 19 Aug 2026 12:52:53 +0000 X-Migadu-Flow: FLOW_OUT Message-ID: Date: Wed, 19 Aug 2026 13:52:46 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 06/12] mm: swap in PMD swap entries as whole THPs during swapoff To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, ying.huang@linux.alibaba.com, baoquan.he@linux.dev, willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, nico.pache@linux.dev, liam@infradead.org, ryan.roberts@arm.com, vbabka@kernel.org, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, kernel-team@meta.com, linmiaohe@huawei.com References: <20260818131202.494754-7-usama.arif@linux.dev> <20260819053803.15868-1-lance.yang@linux.dev> Content-Language: en-US From: Usama Arif In-Reply-To: <20260819053803.15868-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3550B80011 X-Rspam-User: X-Stat-Signature: g17aptojrpasepr9dssaez7uh6jqw54i X-Rspamd-Server: rspam06 X-HE-Tag: 1787143974-413649 X-HE-Meta: U2FsdGVkX18gz4wN3C8HvnbXkK8JcIU19OkVR5jwfiPqitLkejTxkILIcFVo7tNdms1OXtKmtJorqvM0bJrqV9QqorDzVmlNEBArDI1yufTg2+r3/PhMfKQ8lesQ5EQYWW/uRlbbneRsTatpLwm+29shIzoxSv0Kb/4ieDhyacTCKuP/aZmV/1b5J6T11p6heRl6cME/q5Fpr/Ajmog/ncm0ZXGrwXIJtIQigdV+DxtvlvQq/aJiaRZKJpe5vf27ycvG47qDjxe41pAKDr2ZS/P9pgepYPnoMZ3HOOSDmuplDYgHOVkYmW188AdM0anH2jiOytTwUZvkk6qq2eDVbOYa738i7qfNDXLCkRGYfDadHwPVb/5D4m7q/MuK1zJP7TevJ/KAmsBEOj+ItIhvb7CoJ3tFtxeC/wpM37pC6LL5dtC3TonwX5UPfK7fEUKEn6eHouRVP0iNENnwgYk/tQuIq9lE4Z10dNRC/nPv8pTBUnJVUE0jQGiBFiaJHK6InmNtm1JxWuQvIUxVTzsNSrbo1uuBn/qglViVsW0AYrLCbRKjG9y7V0JKrCKEpIqMULmUOIyEsVo7zT48xDUHjrtaDQQmdZBezOxIqHkkrqQs8GRElF6z4h4SnVQYRe1QpzP5+iIyMx2gQIvL+dWViqLCBzPa4N8PhBdRJDishoajunrwTUPYwVp/LWcb08t1FNhgzR3cbB/i1u7hSbaYz5dV929ePL1gxmAoLX+rJNBPuAElSZ7MGHb5pVq6Ee3GuuYqiITH1kv2+KUH4pXQ+KtFLZ07Y8AUBfPoqYUvfDjr5/Dx0uxRp3IVEgT0gEfwfGU7bMD5khPoO2kPvtGjvch+0cV+N/+e+mld2BMtcpKUtCNUqFOdr1ap8pk36WauPo0cjcTR+L0jk4gli7U9ftelxuH9mfi/vrTYQ0xJVeMQcbGNrwSyIozb+A3+PRuGsouiG73V54DHjmd5R1b 2tk02T0T jO4jmrhtWOCcgHL8ecGU5729jmZ/Yg5hZjxHZe25AfbfkSaRk/uflrpF+cshyh+SItLQk2P0mwz570gkuZ/tRzHPQFbb+bz3tQLVuZ3Nh0cfIO7J3UB29e95LOyoUcPde/5qA1+LO7wcB6Ts86+PwHr2HlO1vfJhW/fyp7qzzBHN7NGza0lqV24er1tzlgzOSCdOt0yxPk4vA50HUEumB2ElZx41pu6MTSCKfdFkf8fa/fkODhGMb4c+P5k+xHDxQyFIq2OPIgUmm0c6CTrMpMDD/LXwe4sP5OzUsfB3jMtaC6GE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19/08/2026 06:38, Lance Yang wrote: > +Cc Miaohe > > On Tue, Aug 18, 2026 at 06:09:47AM -0700, Usama Arif wrote: > [...] >> +#ifdef CONFIG_THP_SWAP >> +/* >> + * unuse_pmd - Map a locked folio at PMD granularity during swapoff. >> + * >> + * The caller provides a locked, swapped-in folio. Returns 0 on success >> + * (PMD was mapped). Returns -EAGAIN if the swap cache folio no longer >> + * matches the entry or the PMD changed under the lock (try_to_unuse will >> + * rescan). Returns -EIO if the folio is not uptodate or contains a poisoned >> + * subpage; in that case the PMD is split so unuse_pte_range() can handle >> + * individual pages. >> + */ >> +static int unuse_pmd(struct vm_area_struct *vma, pmd_t *pmd, >> + unsigned long addr, softleaf_t entry, >> + struct folio *folio) >> +{ >> + struct mm_struct *mm = vma->vm_mm; >> + struct page *page; >> + pmd_t new_pmd, old_pmd; >> + spinlock_t *ptl; >> + rmap_t rmap_flags = RMAP_NONE; >> + bool exclusive; >> + >> + if (unlikely(!folio_matches_swap_entry(folio, entry))) >> + return -EAGAIN; >> + >> + if (unlikely(!folio_test_uptodate(folio))) { >> + /* Let PTE fallback reread each slot independently. */ >> + swap_cache_del_folio(folio); >> + __split_huge_pmd(vma, pmd, addr, false); >> + return -EIO; >> + } >> + >> + if (unlikely(folio_contain_hwpoisoned_page(folio))) { >> + /* Let PTE fallback isolate the poisoned subpages. */ >> + __split_huge_pmd(vma, pmd, addr, false); >> + return -EIO; >> + } > > Hmm ... can this miss a poisoned tail? > > memory_failure() sets PageHWPoison(p) before taking folio_lock(), but > PG_has_hwpoisoned is only set later. Since unuse_pmd_entry() holds folio > lock across this check, memory_failure() can be blocked on folio_lock() > with a tail already poisoned, IIUC ... > > folio_contain_hwpoisoned_page() then sees neither a poisoned head nor > PG_has_hwpoisoned, and set_pmd_at() maps that tail through a normal PMD. > > Note that unuse_pte() checks PageHWPoison(page) directly. Should we check > each subpage here as well before installing the PMD? Good catch! I am going to introduce a helper like below and use it in the next revision. Thanks! static inline bool folio_has_hwpoisoned_subpage(struct folio *folio) { unsigned long i; for (i = 0; i < folio_nr_pages(folio); i++) if (PageHWPoison(folio_page(folio, i))) return true; return false; } > >> + >> + page = folio_page(folio, 0); >> + >> + ptl = pmd_lock(mm, pmd); >> + old_pmd = pmdp_get(pmd); >> + >> + if (!pmd_is_swap_entry(old_pmd) || >> + softleaf_from_pmd(old_pmd).val != entry.val) { >> + spin_unlock(ptl); >> + return -EAGAIN; >> + } >> + >> + exclusive = pmd_swp_exclusive(old_pmd); >> + >> + /* >> + * Some architectures may have to restore extra metadata to the folio >> + * when reading from swap. This metadata may be indexed by swap entry >> + * so this must be called before folio_put_swap(). >> + */ >> + arch_swap_restore(folio_swap(entry, folio), folio); >> + >> + add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR); >> + add_mm_counter(mm, MM_SWAPENTS, -HPAGE_PMD_NR); >> + >> + new_pmd = folio_mk_pmd(folio, vma->vm_page_prot); >> + new_pmd = pmd_mkold(new_pmd); >> + if (pmd_swp_soft_dirty(old_pmd)) >> + new_pmd = pmd_mksoft_dirty(new_pmd); >> + if (pmd_swp_uffd(old_pmd)) >> + new_pmd = pmd_mkuffd(new_pmd); >> + if (pmd_swp_uffd(old_pmd) && userfaultfd_rwp(vma)) >> + new_pmd = pmd_modify(new_pmd, PAGE_NONE); >> + >> + if (exclusive) >> + rmap_flags |= RMAP_EXCLUSIVE; >> + >> + folio_get(folio); >> + if (!folio_test_anon(folio)) >> + folio_add_new_anon_rmap(folio, vma, addr, rmap_flags); >> + else >> + folio_add_anon_rmap_pmd(folio, page, vma, addr, rmap_flags); >> + >> + set_pmd_at(mm, addr, pmd, new_pmd); >> + folio_put_swap(folio, NULL); >> + >> + spin_unlock(ptl); >> + >> + folio_free_swap(folio); >> + return 0; >> +} > > [...] > > Cheers, Lance