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Message-ID: <CAMgjq7B4RSDYAJ5aGijqq9cAzC8Jd8TF6gu-gpKjO5=E9a-RbQ@mail.gmail.com>
Date: Mon, 23 Jun 2025 11:35:06 +0800
From: Kairui Song <ryncsn@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Kemeng Shi <shikemeng@...weicloud.com>, Chris Li <chrisl@...nel.org>,
Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm/shmem, swap: improve cached mTHP handling and
fix potential hung
On Mon, Jun 23, 2025 at 11:26 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
> Hi Kairui,
>
> On 2025/6/20 01:55, Kairui Song wrote:
> > From: Kairui Song <kasong@...cent.com>
> >
> > The current swap-in code assumes that, when a swap entry in shmem
> > mapping is order 0, its cached folios (if present) must be order 0
> > too, which turns out not always correct.
> >
> > The problem is shmem_split_large_entry is called before verifying the
> > folio will eventually be swapped in, one possible race is:
> >
> > CPU1 CPU2
> > shmem_swapin_folio
> > /* swap in of order > 0 swap entry S1 */
> > folio = swap_cache_get_folio
> > /* folio = NULL */
> > order = xa_get_order
> > /* order > 0 */
> > folio = shmem_swap_alloc_folio
> > /* mTHP alloc failure, folio = NULL */
> > <... Interrupted ...>
> > shmem_swapin_folio
> > /* S1 is swapped in */
> > shmem_writeout
> > /* S1 is swapped out, folio cached */
> > shmem_split_large_entry(..., S1)
> > /* S1 is split, but the folio covering it has order > 0 now */
> >
> > Now any following swapin of S1 will hang: `xa_get_order` returns 0,
> > and folio lookup will return a folio with order > 0. The
> > `xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will
> > always return false causing swap-in to return -EEXIST.
> >
> > And this looks fragile. So fix this up by allowing seeing a larger folio
> > in swap cache, and check the whole shmem mapping range covered by the
> > swapin have the right swap value upon inserting the folio. And drop
> > the redundant tree walks before the insertion.
> >
> > This will actually improve the performance, as it avoided two redundant
> > Xarray tree walks in the hot path, and the only side effect is that in
> > the failure path, shmem may redundantly reallocate a few folios
> > causing temporary slight memory pressure.
> >
> > And worth noting, it may seems the order and value check before
> > inserting might help reducing the lock contention, which is not true.
> > The swap cache layer ensures raced swapin will either see a swap cache
> > folio or failed to do a swapin (we have SWAP_HAS_CACHE bit even if
> > swap cache is bypassed), so holding the folio lock and checking the
> > folio flag is already good enough for avoiding the lock contention.
> > The chance that a folio passes the swap entry value check but the
> > shmem mapping slot has changed should be very low.
>
> Thanks for fixing the issue. Sadly, I haven't reproduced this issue from
> my previous test cases :(
>
> And I have a question below.
>
> > Cc: stable@...r.kernel.org
> > Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > Reviewed-by: Kemeng Shi <shikemeng@...weicloud.com>
> > ---
> > mm/shmem.c | 30 +++++++++++++++++++++---------
> > 1 file changed, 21 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index eda35be2a8d9..4e7ef343a29b 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -884,7 +884,9 @@ static int shmem_add_to_page_cache(struct folio *folio,
> > pgoff_t index, void *expected, gfp_t gfp)
> > {
> > XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
> > - long nr = folio_nr_pages(folio);
> > + unsigned long nr = folio_nr_pages(folio);
> > + swp_entry_t iter, swap;
> > + void *entry;
> >
> > VM_BUG_ON_FOLIO(index != round_down(index, nr), folio);
> > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> > @@ -896,14 +898,24 @@ static int shmem_add_to_page_cache(struct folio *folio,
> >
> > gfp &= GFP_RECLAIM_MASK;
> > folio_throttle_swaprate(folio, gfp);
> > + swap = iter = radix_to_swp_entry(expected);
> >
> > do {
> > xas_lock_irq(&xas);
> > - if (expected != xas_find_conflict(&xas)) {
> > - xas_set_err(&xas, -EEXIST);
> > - goto unlock;
> > + xas_for_each_conflict(&xas, entry) {
> > + /*
> > + * The range must either be empty, or filled with
> > + * expected swap entries. Shmem swap entries are never
> > + * partially freed without split of both entry and
> > + * folio, so there shouldn't be any holes.
> > + */
> > + if (!expected || entry != swp_to_radix_entry(iter)) {
> > + xas_set_err(&xas, -EEXIST);
> > + goto unlock;
> > + }
> > + iter.val += 1 << xas_get_order(&xas);
> > }
> > - if (expected && xas_find_conflict(&xas)) {
> > + if (expected && iter.val - nr != swap.val) {
> > xas_set_err(&xas, -EEXIST);
> > goto unlock;
> > }
> > @@ -2323,7 +2335,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > error = -ENOMEM;
> > goto failed;
> > }
> > - } else if (order != folio_order(folio)) {
> > + } else if (order > folio_order(folio)) {
> > /*
> > * Swap readahead may swap in order 0 folios into swapcache
> > * asynchronously, while the shmem mapping can still stores
> > @@ -2348,15 +2360,15 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> >
> > swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
> > }
> > + } else if (order < folio_order(folio)) {
> > + swap.val = round_down(swp_type(swap), folio_order(folio));
>
> Why rounding down the swap type? do you mean rounding down the swap offset?
Ouch, right, it should be the value:
swap.val = round_down(swap.val, folio_order(folio));
I messed up the code here during a rebase, let me send a V3 then.
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