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Sun, 06 Jul 2025 04:50:47 -0700 (PDT) MIME-Version: 1.0 References: <20250704181748.63181-1-ryncsn@gmail.com> <20250704181748.63181-5-ryncsn@gmail.com> <452cad4b-e0c7-4792-9272-69199fa52a55@linux.alibaba.com> In-Reply-To: <452cad4b-e0c7-4792-9272-69199fa52a55@linux.alibaba.com> From: Kairui Song Date: Sun, 6 Jul 2025 19:50:10 +0800 X-Gm-Features: Ac12FXyWcjznOL2DsXuvAkXKfhQ5XmunMLI_wpd_eKQiD5XKHdBMZWiZdw2KkfY Message-ID: Subject: Re: [PATCH v4 4/9] mm/shmem, swap: tidy up swap entry splitting To: Baolin Wang Cc: linux-mm@kvack.org, Andrew Morton , Hugh Dickins , Matthew Wilcox , Kemeng Shi , Chris Li , Nhat Pham , Baoquan He , Barry Song , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: itfxr7874kgi5irdgshpfq5dqfrxhzrf X-Rspamd-Queue-Id: ED246140004 X-Rspamd-Server: rspam11 X-Rspam-User: X-HE-Tag: 1751802649-437368 X-HE-Meta: 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 873d4P1v r1w7UgbD8hYpN8AA0gxzOVs2jBaZRm9+d2Px35QTcGmFHeBI2K/BBbi7Yz8iius+WNoL/A1mCrZGaruP54HCpPPIjCf8VSRQeTmju3Ujlx7PDQ0DJlMzLwFcg5a4TtMvPDK5yhJYdSC10zU5ahOPI3/8t8ELO+6GyZaK3O8ojFuVcmhY1ang6OOyIB44FyXIqxUYqn49DkaDQfAdnLOYaWGILyBiNJG7CFEwrQgYVDf+ZSyhB8opl/bj6U+ka2EXhT/3D984UJXV+aRZNX3DXBKSd46ObrjjQYCBx7qk+TRjWt3lmaVliAcNbjgK+RaG6vOLwB69bnIWkenjdCdjS5bkeEEPcWEXx5IrWK398qlIhifG8k94UT99QZIvr6sV0ga5ZTtCK4LGASqIOdifHkGFICw== 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 Sun, Jul 6, 2025 at 11:38=E2=80=AFAM Baolin Wang wrote: > > > > On 2025/7/5 02:17, Kairui Song wrote: > > From: Kairui Song > > > > Instead of keeping different paths of splitting the entry before the > > swap in start, move the entry splitting after the swapin has put > > the folio in swap cache (or set the SWAP_HAS_CACHE bit). This way > > we only need one place and one unified way to split the large entry. > > Whenever swapin brought in a folio smaller than the shmem swap entry, > > split the entry and recalculate the entry and index for verification. > > > > This removes duplicated codes and function calls, reduces LOC, > > and the split is less racy as it's guarded by swap cache now. So it > > will have a lower chance of repeated faults due to raced split. > > The compiler is also able to optimize the coder further: > > > > bloat-o-meter results with GCC 14: > > > > With DEBUG_SECTION_MISMATCH (-fno-inline-functions-called-once): > > ./scripts/bloat-o-meter mm/shmem.o.old mm/shmem.o > > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-82 (-82) > > Function old new delta > > shmem_swapin_folio 2361 2279 -82 > > Total: Before=3D33151, After=3D33069, chg -0.25% > > > > With !DEBUG_SECTION_MISMATCH: > > ./scripts/bloat-o-meter mm/shmem.o.old mm/shmem.o > > add/remove: 0/1 grow/shrink: 1/0 up/down: 949/-750 (199) > > Function old new delta > > shmem_swapin_folio 2878 3827 +949 > > shmem_split_large_entry.isra 750 - -750 > > Total: Before=3D33086, After=3D33285, chg +0.60% > > > > Since shmem_split_large_entry is only called in one place now. The > > compiler will either generate more compact code, or inlined it for > > better performance. > > > > Signed-off-by: Kairui Song > > --- > > mm/shmem.c | 53 +++++++++++++++++++++-------------------------------- > > 1 file changed, 21 insertions(+), 32 deletions(-) > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index e43becfa04b3..217264315842 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > @@ -2266,14 +2266,15 @@ static int shmem_swapin_folio(struct inode *ino= de, pgoff_t index, > > struct address_space *mapping =3D inode->i_mapping; > > struct mm_struct *fault_mm =3D vma ? vma->vm_mm : NULL; > > struct shmem_inode_info *info =3D SHMEM_I(inode); > > + swp_entry_t swap, index_entry; > > struct swap_info_struct *si; > > struct folio *folio =3D NULL; > > bool skip_swapcache =3D false; > > - swp_entry_t swap; > > int error, nr_pages, order, split_order; > > + pgoff_t offset; > > > > VM_BUG_ON(!*foliop || !xa_is_value(*foliop)); > > - swap =3D radix_to_swp_entry(*foliop); > > + swap =3D index_entry =3D radix_to_swp_entry(*foliop); > > *foliop =3D NULL; > > > > if (is_poisoned_swp_entry(swap)) > > @@ -2321,46 +2322,35 @@ static int shmem_swapin_folio(struct inode *ino= de, pgoff_t index, > > } > > > > /* > > - * Now swap device can only swap in order 0 folio, then w= e > > - * should split the large swap entry stored in the pageca= che > > - * if necessary. > > - */ > > - split_order =3D shmem_split_large_entry(inode, index, swa= p, gfp); > > - if (split_order < 0) { > > - error =3D split_order; > > - goto failed; > > - } > > - > > - /* > > - * If the large swap entry has already been split, it is > > + * Now swap device can only swap in order 0 folio, it is > > * necessary to recalculate the new swap entry based on > > - * the old order alignment. > > + * the offset, as the swapin index might be unalgined. > > */ > > - if (split_order > 0) { > > - pgoff_t offset =3D index - round_down(index, 1 <<= split_order); > > - > > + if (order) { > > + offset =3D index - round_down(index, 1 << order); > > swap =3D swp_entry(swp_type(swap), swp_offset(swa= p) + offset); > > } > > > > - /* Here we actually start the io */ > > folio =3D shmem_swapin_cluster(swap, gfp, info, index); > > if (!folio) { > > error =3D -ENOMEM; > > goto failed; > > } > > - } else if (order > folio_order(folio)) { > > + } > > +alloced: > > + if (order > folio_order(folio)) { > > /* > > - * Swap readahead may swap in order 0 folios into swapcac= he > > + * Swapin may get smaller folios due to various reasons: > > + * It may fallback to order 0 due to memory pressure or r= ace, > > + * swap readahead may swap in order 0 folios into swapcac= he > > * asynchronously, while the shmem mapping can still stor= es > > * large swap entries. In such cases, we should split the > > * large swap entry to prevent possible data corruption. > > */ > > - split_order =3D shmem_split_large_entry(inode, index, swa= p, gfp); > > + split_order =3D shmem_split_large_entry(inode, index, ind= ex_entry, gfp); > > if (split_order < 0) { > > - folio_put(folio); > > - folio =3D NULL; > > error =3D split_order; > > - goto failed; > > + goto failed_nolock; > > } > > > > /* > > @@ -2369,15 +2359,13 @@ static int shmem_swapin_folio(struct inode *ino= de, pgoff_t index, > > * the old order alignment. > > */ > > if (split_order > 0) { > > - pgoff_t offset =3D index - round_down(index, 1 <<= split_order); > > - > > + offset =3D index - round_down(index, 1 << split_o= rder); > > swap =3D swp_entry(swp_type(swap), swp_offset(swa= p) + offset); > > Obviously, you should use the original swap value 'index_entry' to > calculate the new swap value. Thanks, good catch. > > With the following fix, you can add: > Reviewed-by: Baolin Wang > Tested-by: Baolin Wang > > diff --git a/mm/shmem.c b/mm/shmem.c > index d530df550f7f..1e8422ac863e 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -2361,7 +2361,7 @@ static int shmem_swapin_folio(struct inode *inode, > pgoff_t index, > */ > if (split_order > 0) { > offset =3D index - round_down(index, 1 << > split_order); > - swap =3D swp_entry(swp_type(swap), > swp_offset(swap) + offset); > + swap =3D swp_entry(swp_type(swap), > swp_offset(index_swap) + offset); > } > } else if (order < folio_order(folio)) { > swap.val =3D round_down(swap.val, 1 << folio_order(folio= )); > >