* + mm-streamline-cow-logic-in-do_swap_page.patch added to -mm tree
@ 2022-01-31 19:43 akpm
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From: akpm @ 2022-01-31 19:43 UTC (permalink / raw)
To: aarcange, david, ddutile, guro, hch, hughd, jack, jannh, jgg,
jhubbard, kirill.shutemov, mhocko, mike.kravetz, mm-commits,
nadav.amit, oleg, peterx, riel, rientjes, rppt, shakeelb,
shy828301, torvalds, vbabka, willy, zhangliang5
The patch titled
Subject: mm: streamline COW logic in do_swap_page()
has been added to the -mm tree. Its filename is
mm-streamline-cow-logic-in-do_swap_page.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-streamline-cow-logic-in-do_swap_page.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-streamline-cow-logic-in-do_swap_page.patch
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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm: streamline COW logic in do_swap_page()
Currently we have a different COW logic when:
* triggering a read-fault to swapin first and then trigger a write-fault
-> do_swap_page() + do_wp_page()
* triggering a write-fault to swapin
-> do_swap_page() + do_wp_page() only if we fail reuse in do_swap_page()
The COW logic in do_swap_page() is different than our reuse logic in
do_wp_page(). The COW logic in do_wp_page() -- page_count() == 1 -- makes
currently sure that we certainly don't have a remaining reference, e.g.,
via GUP, on the target page we want to reuse: if there is any unexpected
reference, we have to copy to avoid information leaks.
As do_swap_page() behaves differently, in environments with swap enabled
we can currently have an unintended information leak from the parent to
the child, similar as known from CVE-2020-29374:
1. Parent writes to anonymous page
-> Page is mapped writable and modified
2. Page is swapped out
-> Page is unmapped and replaced by swap entry
3. fork()
-> Swap entries are copied to child
4. Child pins page R/O
-> Page is mapped R/O into child
5. Child unmaps page
-> Child still holds GUP reference
6. Parent writes to page
-> Page is reused in do_swap_page()
-> Child can observe changes
Exchanging 2. and 3. should have the same effect.
Let's apply the same COW logic as in do_wp_page(), conditionally trying to
remove the page from the swapcache after freeing the swap entry, however,
before actually mapping our page. We can change the order now that we use
try_to_free_swap(), which doesn't care about the mapcount, instead of
reuse_swap_page().
To handle references from the LRU pagevecs, conditionally drain the local
LRU pagevecs when required, however, don't consider the page_count() when
deciding whether to drain to keep it simple for now.
Link: https://lkml.kernel.org/r/20220131162940.210846-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/memory.c | 55 +++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 43 insertions(+), 12 deletions(-)
--- a/mm/memory.c~mm-streamline-cow-logic-in-do_swap_page
+++ a/mm/memory.c
@@ -3497,6 +3497,25 @@ static vm_fault_t remove_device_exclusiv
return 0;
}
+static inline bool should_try_to_free_swap(struct page *page,
+ struct vm_area_struct *vma,
+ unsigned int fault_flags)
+{
+ if (!PageSwapCache(page))
+ return false;
+ if (mem_cgroup_swap_full(page) || (vma->vm_flags & VM_LOCKED) ||
+ PageMlocked(page))
+ return true;
+ /*
+ * If we want to map a page that's in the swapcache writable, we
+ * have to detect via the refcount if we're really the exclusive
+ * user. Try freeing the swapcache to get rid of the swapcache
+ * reference only in case it's likely that we'll be the exlusive user.
+ */
+ return (fault_flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+ page_count(page) == 2;
+}
+
/*
* We enter with non-exclusive mmap_lock (to exclude vma changes,
* but allow concurrent faults), and pte mapped but not yet locked.
@@ -3638,6 +3657,16 @@ vm_fault_t do_swap_page(struct vm_fault
page = swapcache;
goto out_page;
}
+
+ /*
+ * If we want to map a page that's in the swapcache writable, we
+ * have to detect via the refcount if we're really the exclusive
+ * owner. Try removing the extra reference from the local LRU
+ * pagevecs if required.
+ */
+ if ((vmf->flags & FAULT_FLAG_WRITE) && page == swapcache &&
+ !PageKsm(page) && !PageLRU(page))
+ lru_add_drain();
}
cgroup_throttle_swaprate(page, GFP_KERNEL);
@@ -3656,19 +3685,25 @@ vm_fault_t do_swap_page(struct vm_fault
}
/*
- * The page isn't present yet, go ahead with the fault.
- *
- * Be careful about the sequence of operations here.
- * To get its accounting right, reuse_swap_page() must be called
- * while the page is counted on swap but not yet in mapcount i.e.
- * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
- * must be called after the swap_free(), or it will never succeed.
+ * Remove the swap entry and conditionally try to free up the swapcache.
+ * We're already holding a reference on the page but haven't mapped it
+ * yet.
*/
+ swap_free(entry);
+ if (should_try_to_free_swap(page, vma, vmf->flags))
+ try_to_free_swap(page);
inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
pte = mk_pte(page, vma->vm_page_prot);
- if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
+
+ /*
+ * Same logic as in do_wp_page(); however, optimize for fresh pages
+ * that are certainly not shared because we just allocated them without
+ * exposing them to the swapcache.
+ */
+ if ((vmf->flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+ (page != swapcache || page_count(page) == 1)) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
vmf->flags &= ~FAULT_FLAG_WRITE;
ret |= VM_FAULT_WRITE;
@@ -3694,10 +3729,6 @@ vm_fault_t do_swap_page(struct vm_fault
set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
- swap_free(entry);
- if (mem_cgroup_swap_full(page) ||
- (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
- try_to_free_swap(page);
unlock_page(page);
if (page != swapcache && swapcache) {
/*
_
Patches currently in -mm which might be from david@redhat.com are
mm-optimize-do_wp_page-for-exclusive-pages-in-the-swapcache.patch
mm-optimize-do_wp_page-for-fresh-pages-in-local-lru-pagevecs.patch
mm-slightly-clarify-ksm-logic-in-do_swap_page.patch
mm-streamline-cow-logic-in-do_swap_page.patch
mm-huge_memory-streamline-cow-logic-in-do_huge_pmd_wp_page.patch
mm-khugepaged-remove-reuse_swap_page-usage.patch
mm-swapfile-remove-stale-reuse_swap_page.patch
mm-huge_memory-remove-stale-page_trans_huge_mapcount.patch
mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd.patch
proc-vmcore-fix-possible-deadlock-on-concurrent-mmap-and-read.patch
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