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Howlett , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com, Usama Arif Subject: [PATCH v8 25/30] mm: handle PMD swap entries in MADV_WILLNEED Date: Fri, 2 Oct 2026 02:52:39 -0700 Message-ID: <20261002095503.3585565-26-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261002095503.3585565-1-usama.arif@linux.dev> References: <20261002095503.3585565-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: d7o8i9qei9xdxyi7mazxe6an69tqkb3a X-Rspamd-Queue-Id: BF8C4140003 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790935045-24239 X-HE-Meta: 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 ctwirVjV ov64ozUPfqKmOA1CwA+rl5OKLq+tqR2HUB2kxRaP+KLSysT8LoVxV47c1/XwT3a7SvsxqywptxKvj4RlSiZbiyUhDQbyqF9eo3+XMh7uGOjyeUTIvCEOWE6A4+S5YH8SoRAusqq4gIkQjohQsEZPgnKmbMsK8fF20m/5JZuJ9vGTr++Hbmoc665dkcxCqkR8HRf/yS1AD94vuDYLXFzPGKnguH8wajlYdgg09ClfVxtiW+RV774febGpsMH/RXVhFfcK0hDdT6EqI6KRv6EsgnqAeNw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: swapin_walk_pmd_entry() walks PTEs and cannot descend into a non-present PMD, so MADV_WILLNEED is a no-op on a PMD swap entry. Reading the slots back one at a time would not be much better: order-0 readahead populates per-page swap-cache state, and the fault that follows would then have to split the entry the prefetch was supposed to help. Prefetch the whole range as one PMD-order folio instead, leaving the entry in place so the eventual fault still takes do_huge_pmd_swap_page(). Check the THP policy first: the range was PMD-mapped when it was swapped out, but MADV_NOHUGEPAGE or the sysfs knob may have changed since, and reading at PMD order would then hand do_swap_page() a PMD-sized folio to map with PTEs. A split cache, per-page zswap state, or a failed PMD-order read or allocation falls back to PTEs. Leaving the entry alone would prefetch nothing at all, while the PTE path can still read the slots at order 0. A folio that failed to read is dropped from the swap cache first, so the PTE retry re-reads each slot rather than every later fault returning SIGBUS. Only split if the PMD still looks like the entry that was observed - the decision is taken after the PMD lock has been dropped, so a racing fault may have swapped the range back in as a THP. The test is lockless, so it narrows that window rather than closing it. Signed-off-by: Usama Arif --- mm/madvise.c | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 139 insertions(+) diff --git a/mm/madvise.c b/mm/madvise.c index 64019668d69fb..ec1cb1b71070a 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -33,6 +33,7 @@ #include #include #include +#include #include @@ -194,6 +195,108 @@ static int madvise_update_vma(vm_flags_t new_flags, } #ifdef CONFIG_SWAP +/* + * Prefetch a whole PMD swap entry as one PMD-order folio. + * + * Called with the PMD lock held; always drops it. Returns true when the + * caller should ask the walker to retry so the PTE path can handle the + * covered slots individually. + */ +static bool swapin_pmd_swap_entry(struct vm_area_struct *vma, pmd_t *pmd, + unsigned long addr, softleaf_t entry, + spinlock_t *ptl) +{ + struct vm_fault vmf = { + .vma = vma, + .address = addr, + .real_address = addr, + .pmd = pmd, + }; + enum swap_pmd_cache cache_state; + struct swap_info_struct *si; + struct folio *folio; + bool split = false; + + /* + * The range was PMD-mapped when it was swapped out, but the policy may + * have changed since: MADV_NOHUGEPAGE, or the sysfs knob. Reading it + * back at PMD order would then hand do_swap_page() a PMD-sized folio to + * map with PTEs, which is exactly what the policy forbids. Split now + * and let the PTE path prefetch at order 0 - the next fault would split + * the entry anyway, so nothing is lost that the VMA still permits. + */ + if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_PAGEFAULT, + HPAGE_PMD_ORDER)) { + spin_unlock(ptl); + return true; + } + + cache_state = swap_pmd_cache_lookup(entry, &folio); + if (cache_state == SWAP_PMD_CACHE_HUGE) { + /* Already cached as one PMD-sized folio, nothing to do. */ + folio_put(folio); + spin_unlock(ptl); + return false; + } + if (cache_state == SWAP_PMD_CACHE_SPLIT || + zswap_is_present(entry, HPAGE_PMD_NR)) { + spin_unlock(ptl); + return true; + } + + /* + * Pin the swap device under the PMD lock so the PMD-swap-entry + * observation keeps the entry valid for swapin_sync(). + */ + si = get_swap_device(entry); + spin_unlock(ptl); + if (IS_ERR_OR_NULL(si)) + return false; + + folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE, BIT(HPAGE_PMD_ORDER), + &vmf, NULL, 0); + + /* + * Fall back to PTE-order swapin: a PMD-order failure does not mean + * that individual slots cannot be read. + */ + if (IS_ERR_OR_NULL(folio)) { + split = true; + goto out; + } + + if (folio_nr_pages(folio) != HPAGE_PMD_NR) { + split = true; + goto out_put; + } + + /* + * A trylock only succeeds once the read has completed, so this never + * blocks MADV_WILLNEED on in-flight I/O. A read that failed - a + * PMD-order zswap load that found per-page state, or an I/O error - + * leaves the folio clean and not uptodate. Drop it from the swap cache + * so the PTE retry reads each slot again; leaving it there would make + * the next fault return VM_FAULT_SIGBUS. Another thread may have + * removed it already, so revalidate the association first. + */ + if (!folio_trylock(folio)) + goto out_put; + + if (!folio_test_uptodate(folio)) { + if (folio_matches_swap_entry(folio, entry)) + swap_cache_del_folio(folio); + split = true; + } + folio_unlock(folio); + +out_put: + folio_put(folio); +out: + /* Keep the device pinned until the last use of @entry. */ + put_swap_device(si); + return split; +} + static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start, unsigned long end, struct mm_walk *walk) { @@ -203,6 +306,41 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start, spinlock_t *ptl; unsigned long addr; + ptl = pmd_trans_huge_lock(pmd, vma); + if (ptl) { + pmd_t pmdval = *pmd; + + if (pmd_is_swap_entry(pmdval)) { + /* swapin_pmd_swap_entry() always drops the PMD lock. */ + if (!swapin_pmd_swap_entry(vma, pmd, start, + softleaf_from_pmd(pmdval), + ptl)) + goto ret; + /* + * Only split if this still looks like the entry we + * observed. The fallback was decided after the PMD lock + * was dropped, so a racing fault may have swapped the + * range back in as a THP, and splitting that would + * demote a perfectly good huge mapping for an advisory + * hint. The test is lockless, so it narrows that window + * rather than closing it. + */ + if (pmd_same(pmdval, pmdp_get_lockless(pmd))) { + __split_huge_pmd(vma, pmd, start); + walk->action = ACTION_AGAIN; + goto ret; + } + /* + * Somebody else changed the PMD. Leave it alone and let + * the PTE loop below deal with whatever is there now; + * it simply finds no page table if the range came back + * as a THP. + */ + } else { + spin_unlock(ptl); + } + } + for (addr = start; addr < end; addr += PAGE_SIZE) { pte_t pte; softleaf_t entry; @@ -231,6 +369,7 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start, if (ptep) pte_unmap_unlock(ptep, ptl); swap_read_submit(&ctx); +ret: cond_resched(); return 0; -- 2.53.0-Meta