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Fri, 05 Sep 2025 23:32:46 -0700 (PDT) MIME-Version: 1.0 References: <20250905191357.78298-1-ryncsn@gmail.com> <20250905191357.78298-6-ryncsn@gmail.com> In-Reply-To: From: Kairui Song Date: Sat, 6 Sep 2025 14:32:10 +0800 X-Gm-Features: Ac12FXxno4Q-NpwwKgRz1yo_NwUomWZKwH5I0eRlSUqSUkTlzqqlpVCECnitMLs Message-ID: Subject: Re: [PATCH v2 05/15] mm, swap: always lock and check the swap cache folio before use To: Chris Li Cc: linux-mm@kvack.org, Andrew Morton , Matthew Wilcox , Hugh Dickins , Barry Song , Baoquan He , Nhat Pham , Kemeng Shi , Baolin Wang , Ying Huang , Johannes Weiner , David Hildenbrand , Yosry Ahmed , Lorenzo Stoakes , Zi Yan , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 0749FA000A X-Stat-Signature: r6brrugfnafywdt5yee7eygx37uhxd9h X-Rspam-User: X-HE-Tag: 1757140367-605443 X-HE-Meta: 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 EI7yLhrt bVt3hrS+gYUs06FNDAL6mmNaDYJawjdtEETkYGbGDmtFbJjxEzg5ts+vUJkUFSLDaI6eJoE6sXUKVSCOkLZxY+B6HtW6S7dSw8r+6vRYC1A37pnjjzL3eQRretiRYO7Q+qWiXnYVIK7lJEzdHSeQQ+hM06MKCt0g0PC2f+ZP5i7uBKrWBiqpgwxdo/6xQGQcEp9bE+9OiSC5DwWuoeqsDA9IR1DntZNGxwAQK4q4fuYlzOdnuq+mC1ckwvd7cM6unElmw7MDaWmKvkkEtjm9bDYniBTHcMqzxSoen/v3IUO6yBFAO/A6Tcrt5/Np+Yv8O3i+bph311yW2NPpErD/iQ+/aSbyHsjNxzZVpO6RHkKJPuH8f+NfZmlmQvWNyMOy92c1zK+Ih4Qboou9Y7noKPbnsWLLNZNDQoTzPNwj31eIadZs= 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: List-Subscribe: List-Unsubscribe: On Sat, Sep 6, 2025 at 11:51=E2=80=AFAM Chris Li wrote: > > Looks correct to me. > > Acked-by: Chris Li Thanks. > > With some nitpick follows, > > On Fri, Sep 5, 2025 at 12:14=E2=80=AFPM Kairui Song wr= ote: > > > > From: Kairui Song > > > > Swap cache lookup only increases the reference count of the returned > > folio. That's not enough to ensure a folio is stable in the swap > > cache, so the folio could be removed from the swap cache at any > > time. The caller should always lock and check the folio before using it= . > > > > We have just documented this in kerneldoc, now introduce a helper for s= wap > > cache folio verification with proper sanity checks. > > > > Also, sanitize a few current users to use this convention and the new > > helper for easier debugging. They were not having observable problems > > yet, only trivial issues like wasted CPU cycles on swapoff or > > reclaiming. They would fail in some other way, but it is still better t= o > > always follow this convention to make things robust and make later > > commits easier to do. > > > > Signed-off-by: Kairui Song > > --- > > mm/memory.c | 3 +-- > > mm/swap.h | 24 ++++++++++++++++++++++++ > > mm/swap_state.c | 7 +++++-- > > mm/swapfile.c | 10 +++++++--- > > 4 files changed, 37 insertions(+), 7 deletions(-) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 94a5928e8ace..5808c4ef21b3 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -4748,8 +4748,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > * swapcache, we need to check that the page's swap has= not > > * changed. > > */ > > - if (unlikely(!folio_test_swapcache(folio) || > > - page_swap_entry(page).val !=3D entry.val)) > > + if (unlikely(!folio_matches_swap_entry(folio, entry))) > > goto out_page; > > > > if (unlikely(PageHWPoison(page))) { > > diff --git a/mm/swap.h b/mm/swap.h > > index efb6d7ff9f30..a69e18b12b45 100644 > > --- a/mm/swap.h > > +++ b/mm/swap.h > > @@ -52,6 +52,25 @@ static inline pgoff_t swap_cache_index(swp_entry_t e= ntry) > > return swp_offset(entry) & SWAP_ADDRESS_SPACE_MASK; > > } > > > > +/** > > + * folio_matches_swap_entry - Check if a folio matches a given swap en= try. > > + * @folio: The folio. > > + * @entry: The swap entry to check against. > > + * > > + * Context: The caller should have the folio locked to ensure it's sta= ble > > + * and nothing will move it in or out of the swap cache. > > + * Return: true or false. > > + */ > > +static inline bool folio_matches_swap_entry(const struct folio *folio, > > + swp_entry_t entry) > > +{ > > + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); > > + if (!folio_test_swapcache(folio)) > > + return false; > > + VM_WARN_ON_ONCE_FOLIO(!IS_ALIGNED(folio->swap.val, folio_nr_pag= es(folio)), folio); > > You should cache the folio->swap.val into a local register value. > Because WARN_ON_ONCE I think the compiler has no choice but to load it > twice? Haven't verified it myself. > > There is no downside in compiler point of view using more local > variables, the compiler generates an internal version of the local > variable equivalent anyway. > > > + return folio->swap.val =3D=3D round_down(entry.val, folio_nr_pa= ges(folio)); > > Same for folio_nr_pages(folio), you should cache it. The function will > look less busy. That's a very good idea, that should also reduce line length so it is easier to read.