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Shutemov" , Yang Shi , David Hildenbrand , Hugh Dickins , linux-kernel@vger.kernel.org, David Stevens Subject: [PATCH 2/2] mm/khugepaged: skip shmem with userfaultfd Date: Tue, 14 Feb 2023 16:57:10 +0900 Message-Id: <20230214075710.2401855-2-stevensd@google.com> X-Mailer: git-send-email 2.39.1.581.gbfd45094c4-goog In-Reply-To: <20230214075710.2401855-1-stevensd@google.com> References: <20230214075710.2401855-1-stevensd@google.com> MIME-Version: 1.0 X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4C1EBA0002 X-Stat-Signature: gdcnzndub4ico3w17yfn6ntjohb8ucbi X-HE-Tag: 1676361445-798362 X-HE-Meta: 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 1hmIT5ic C8CpvqVWPshi+aeZ0MYQPYIIwRnQdJXDU66aCWUHplrpSRdMeTLdDHmqUUjoLgSQZlHevaUAUpn9jVtV5+ykNy9xwSlRdDdrSwGfuYV/x2ZxDTNJfPgyw7mdmQZrGwak+eZ8Qnru/YR6P6wnWLNTPKclGtfpxS1IrGdBhX4W0lOrUMZXUybzAuZI+6BXvCpeQY18gfFn/Cbv5syVZmPkBxQTj/Zcel1AAaDw0gumf13pH5byRqiBG4p9NP3N+o/OHT1jSFS+S+Lgaq9FEacrT5K+FGCrxNXG7mWgaYFpCKFeFLse56n8mCW4bs2+9UoLBB6CC X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: David Stevens Make sure that collapse_file respects any userfaultfds registered with MODE_MISSING. If userspace has any such userfaultfds registered, then for any page which it knows to be missing, it may expect a UFFD_EVENT_PAGEFAULT. This means collapse_file needs to take care when collapsing a shmem range would result in replacing an empty page with a THP, so that it doesn't break userfaultfd. Synchronization when checking for userfaultfds in collapse_file is tricky because the mmap locks can't be used to prevent races with the registration of new userfaultfds. Instead, we provide synchronization by ensuring that userspace cannot observe the fact that pages are missing before we check for userfaultfds. Although this allows registration of a userfaultfd to race with collapse_file, it ensures that userspace cannot observe any pages transition from missing to present after such a race. This makes such a race indistinguishable to the collapse occurring immediately before the userfaultfd registration. The first step to provide this synchronization is to stop filling gaps during the loop iterating over the target range, since the page cache lock can be dropped during that loop. The second step is to fill the gaps with XA_RETRY_ENTRY after the page cache lock is acquired the final time, to avoid races with accesses to the page cache that only take the RCU read lock. This fix is targeted at khugepaged, but the change also applies to MADV_COLLAPSE. MADV_COLLAPSE on a range with a userfaultfd will now return EBUSY if there are any missing pages (instead of succeeding on shmem and returning EINVAL on anonymous memory). There is also now a window during MADV_COLLAPSE where a fault on a missing page will cause the syscall to fail with EAGAIN. The fact that intermediate page cache state can no longer be observed before the rollback of a failed collapse is also technically a userspace-visible change (via at least SEEK_DATA and SEEK_END), but it is exceedingly unlikely that anything relies on being able to observe that transient state. Signed-off-by: David Stevens --- mm/khugepaged.c | 66 +++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 58 insertions(+), 8 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index b648f1053d95..8c2e2349e883 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -55,6 +55,7 @@ enum scan_result { SCAN_CGROUP_CHARGE_FAIL, SCAN_TRUNCATED, SCAN_PAGE_HAS_PRIVATE, + SCAN_PAGE_FILLED, }; #define CREATE_TRACE_POINTS @@ -1725,8 +1726,8 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff, * - allocate and lock a new huge page; * - scan page cache replacing old pages with the new one * + swap/gup in pages if necessary; - * + fill in gaps; * + keep old pages around in case rollback is required; + * - finalize updates to the page cache; * - if replacing succeeds: * + copy data over; * + free old pages; @@ -1805,13 +1806,12 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, result = SCAN_TRUNCATED; goto xa_locked; } - xas_set(&xas, index); + xas_set(&xas, index + 1); } if (!shmem_charge(mapping->host, 1)) { result = SCAN_FAIL; goto xa_locked; } - xas_store(&xas, hpage); nr_none++; continue; } @@ -1970,6 +1970,56 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, put_page(page); goto xa_unlocked; } + + if (nr_none) { + struct vm_area_struct *vma; + int nr_none_check = 0; + + xas_unlock_irq(&xas); + i_mmap_lock_read(mapping); + xas_lock_irq(&xas); + + xas_set(&xas, start); + for (index = start; index < end; index++) { + if (!xas_next(&xas)) { + xas_store(&xas, XA_RETRY_ENTRY); + nr_none_check++; + } + } + + if (nr_none != nr_none_check) { + result = SCAN_PAGE_FILLED; + goto immap_locked; + } + + /* + * If userspace observed a missing page in a VMA with an armed + * userfaultfd, then it might expect a UFFD_EVENT_PAGEFAULT for + * that page, so we need to roll back to avoid suppressing such + * an event. Any userfaultfds armed after this point will not be + * able to observe any missing pages due to the previously + * inserted retry entries. + */ + vma_interval_tree_foreach(vma, &mapping->i_mmap, start, start) { + if (userfaultfd_missing(vma)) { + result = SCAN_EXCEED_NONE_PTE; + goto immap_locked; + } + } + +immap_locked: + i_mmap_unlock_read(mapping); + if (result != SCAN_SUCCEED) { + xas_set(&xas, start); + for (index = start; index < end; index++) { + if (xas_next(&xas) == XA_RETRY_ENTRY) + xas_store(&xas, NULL); + } + + goto xa_locked; + } + } + nr = thp_nr_pages(hpage); if (is_shmem) @@ -2068,15 +2118,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, } xas_set(&xas, start); - xas_for_each(&xas, page, end - 1) { + end = index; + for (index = start; index < end; index++) { + xas_next(&xas); page = list_first_entry_or_null(&pagelist, struct page, lru); if (!page || xas.xa_index < page->index) { - if (!nr_none) - break; nr_none--; - /* Put holes back where they were */ - xas_store(&xas, NULL); continue; } @@ -2592,11 +2640,13 @@ static int madvise_collapse_errno(enum scan_result r) case SCAN_ALLOC_HUGE_PAGE_FAIL: return -ENOMEM; case SCAN_CGROUP_CHARGE_FAIL: + case SCAN_EXCEED_NONE_PTE: return -EBUSY; /* Resource temporary unavailable - trying again might succeed */ case SCAN_PAGE_LOCK: case SCAN_PAGE_LRU: case SCAN_DEL_PAGE_LRU: + case SCAN_PAGE_FILLED: return -EAGAIN; /* * Other: Trying again likely not to succeed / error intrinsic to