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From: Baoquan He To: Christoph Hellwig Cc: akpm@linux-foundation.org, chrisl@kernel.org, usama.arif@linux.dev, kasong@tencent.com, nphamcs@gmail.com, shikemeng@huaweicloud.com, youngjun.park@lge.com, linux-mm@kvack.org Subject: Re: [PATCH 7/8] mm/swap: remove SWP_FS_OPS Message-ID: References: <20260601113449.3464734-1-hch@lst.de> <20260601113449.3464734-8-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260601113449.3464734-8-hch@lst.de> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 0282F140006 X-Stat-Signature: ga9enjnjspix9dkzi6ytari81bxwqez7 X-HE-Tag: 1780636910-453602 X-HE-Meta: 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 UA0HRFDX T7QM8B9ztB2ppVHBcOoSCyHHxEJEbAb5SM3TTwji+WflwhgV+mD/5FcPjh9qJvw8Qzrh2oQOi8U9t0WNQwoovNaow6z7YB+CtNXc8FwKjzWCVyIvFpa9TqTnjXqMXthVNELR+ySZxqR0mMzJTPwgEp66V05BdjRWTY7AVW3IDBDj5c2e8WQMxKUQlGrgfU+Bxz6dI9XzhaaatV9lGIaERaFh1p6e/5g05VagZwDfbT4A7B5GEoOcjQVFVEpVkvNpA18kaDmS1zLOu7eZyuBXQ7dYCYwwDa3Xd0iR7MWDzxuWNJic= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 06/01/26 at 01:34pm, Christoph Hellwig wrote: ...snip... > diff --git a/mm/swap.h b/mm/swap.h > index 5119feff0e93..edb512e619ee 100644 > --- a/mm/swap.h > +++ b/mm/swap.h > @@ -84,7 +84,11 @@ struct swap_io_ctx { > struct swap_info_struct *sis; > }; > > +#define SWAP_OPS_F_NOFS (1U << 0) > + The name of this flag and its usage are a little weird, surely the old one SWP_FS_OPS is similar. Do you think if we need add some document as below to make it clearer? Personal opinion and just for reference. diff --git a/mm/swap.h b/mm/swap.h index edb512e619ee..3d978382a7d5 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -84,6 +84,18 @@ struct swap_io_ctx { struct swap_info_struct *sis; }; +/* + * SWAP_OPS_F_NOFS: swap operations that may recurse into the filesystem + * (e.g. NFS, CIFS). When set, callers (specifically reclaim) must operate + * under GFP_NOFS / !__GFP_FS to avoid deadlock. When clear (block-device + * swap), the swap I/O path does not touch the filesystem, so reclaim may + * safely enter the filesystem even with only __GFP_IO. + * + * In may_enter_fs() this appears as a double negative (!SWAP_OPS_F_NOFS) + * which reads as "not a nofs swap => may enter fs". This is intentional: + * it is bdev swap (the common case) that allows filesystem re-entry, while + * filesystem-based swap explicitly forbids it. + */ #define SWAP_OPS_F_NOFS (1U << 0) struct swap_ops { diff --git a/mm/vmscan.c b/mm/vmscan.c index 2d44ebfebdea..a92956de5e10 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1039,7 +1039,9 @@ static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask) return true; /* * We can "enter_fs" for swap-cache with only __GFP_IO unless backed by - * a swapfile that requires GFP_NOFS I/O. + * a swapfile that requires GFP_NOFS I/O (i.e. filesystem-based swap + * like NFS/CIFS, which sets SWAP_OPS_F_NOFS). Block-device swap + * never touches the filesystem, so it is safe to enter fs here. */ if (folio_test_swapcache(folio) && (gfp_mask & __GFP_IO) && !(__swap_entry_to_info(folio->swap)->ops->flags & SWAP_OPS_F_NOFS)) > struct swap_ops { > + unsigned int flags; > + > bool (*can_merge)(struct folio *folio, struct folio *prev_folio, > size_t prev_folio_size, int rw); > void (*submit_write)(struct swap_io_ctx *ctx); > @@ -335,11 +339,6 @@ struct folio *swapin_sync(swp_entry_t entry, gfp_t flag, unsigned long orders, > void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma, > unsigned long addr); > > -static inline unsigned int folio_swap_flags(struct folio *folio) > -{ > - return __swap_entry_to_info(folio->swap)->flags; > -} > - > #else /* CONFIG_SWAP */ > static inline struct swap_cluster_info *swap_cluster_lock( > struct swap_info_struct *si, pgoff_t offset, bool irq) > @@ -470,16 +469,9 @@ static inline void __swap_cache_replace_folio(struct swap_cluster_info *ci, > struct folio *old, struct folio *new) > { > } > - > -static inline unsigned int folio_swap_flags(struct folio *folio) > -{ > - return 0; > -} > - > #endif /* CONFIG_SWAP */ > > extern const struct swap_ops swap_bdev_ops; > -extern const struct swap_ops swap_fs_ops; > > int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio, > struct list_head *folio_list); > diff --git a/mm/swapfile.c b/mm/swapfile.c > index a670635e0fbe..284eebc40a70 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -2858,8 +2858,6 @@ static int setup_swap_extents(struct swap_info_struct *sis, > ret = mapping->a_ops->swap_activate(sis, swap_file, span); > if (ret < 0) > return ret; > - if (sis->flags & SWP_FS_OPS) > - sis->ops = &swap_fs_ops; > sis->flags |= SWP_ACTIVATED; > return ret; > } > diff --git a/mm/vmscan.c b/mm/vmscan.c > index c43177a8e4dd..2d44ebfebdea 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1037,16 +1037,14 @@ static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask) > { > if (gfp_mask & __GFP_FS) > return true; > - if (!folio_test_swapcache(folio) || !(gfp_mask & __GFP_IO)) > - return false; > /* > - * We can "enter_fs" for swap-cache with only __GFP_IO > - * providing this isn't SWP_FS_OPS. > - * ->flags can be updated non-atomically, > - * but that will never affect SWP_FS_OPS, so the data_race > - * is safe. > + * We can "enter_fs" for swap-cache with only __GFP_IO unless backed by > + * a swapfile that requires GFP_NOFS I/O. > */ > - return !data_race(folio_swap_flags(folio) & SWP_FS_OPS); > + if (folio_test_swapcache(folio) && (gfp_mask & __GFP_IO) && > + !(__swap_entry_to_info(folio->swap)->ops->flags & SWAP_OPS_F_NOFS)) > + return true; > + return false; > } > > /* > -- > 2.53.0 >