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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 v9 16/32] mm: zswap: reject high-order swap cache allocations backed by zswap Date: Thu, 5 Nov 2026 10:39:00 -0800 Message-ID: <20261105183940.2236717-17-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261105183940.2236717-1-usama.arif@linux.dev> References: <20261105183940.2236717-1-usama.arif@linux.dev> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit zswap stores swapped-out pages one at a time, so a range of swap slots can be split between zswap entries and the backing device. A high-order swap-cache folio over such a range cannot be read back in one go: the disk read would return stale or zero data for every slot zswap owns. Reject that in __swap_cache_alloc(). Do the check after the folio is in the swap cache: from that point zswap can neither start storing nor start writing back any slot in the range, so the answer is stable. Callers already fall back to order-0, which zswap_load() handles per page. Co-developed-by: Alexandre Ghiti Signed-off-by: Alexandre Ghiti Signed-off-by: Usama Arif --- mm/swap_state.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/mm/swap_state.c b/mm/swap_state.c index a8a486b0403ab..f41e740a91b87 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -12,6 +12,7 @@ #include #include #include +#include #include #include #include @@ -482,6 +483,17 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci, __swap_cache_do_add_folio(ci, folio, entry); spin_unlock(&ci->lock); + /* + * Now that the folio is in the swap cache, zswap can no longer start + * storing or writing back any slot in the range, so this is a stable + * answer. Reject a high-order allocation over a range that already + * has per-page zswap entries. + */ + if (order && zswap_is_present(entry, nr_pages)) { + err = -EBUSY; + goto delete_folio; + } + if (mem_cgroup_swapin_charge_folio(folio, memcg_id, vmf ? vmf->vma->vm_mm : NULL, gfp)) { err = -ENOMEM; -- 2.53.0-Meta