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Thu, 17 Jul 2025 17:44:23 +0800 Message-ID: <853a5211-cdab-4bdf-b0c4-8092dd943ff5@linux.alibaba.com> Date: Thu, 17 Jul 2025 17:44:23 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new 2/2] mm/shmem: writeout free swap if swap_writeout() reactivates To: Hugh Dickins , Andrew Morton Cc: Baoquan He , Barry Song <21cnbao@gmail.com>, Chris Li , David Rientjes , Kairui Song , Kemeng Shi , Shakeel Butt , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <87beaec6-a3b0-ce7a-c892-1e1e5bd57aa3@google.com> <5c911f7a-af7a-5029-1dd4-2e00b66d565c@google.com> From: Baolin Wang In-Reply-To: <5c911f7a-af7a-5029-1dd4-2e00b66d565c@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 151CC4000A X-Rspam-User: X-Rspamd-Server: rspam09 X-Stat-Signature: c76g5ykdwz63pinw6zc7c9qcr9hkkt3n X-HE-Tag: 1752745467-544868 X-HE-Meta: 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 ZpXQ39pT stdejP0020XflYJCcNdtBhhSP9aA0Sn0hYMBLgAcPJUxVT0PqZATBLOdnjRBqK17b09OSCTve5I6qbnX1rvHDf+BHrJ/rGc5pc1qtsFK1AII/gpwv3JZg38vJ3+l3rk+fre+4b/OGymR73VfOIZo/HyieHT8RT2NJhp4Lrw9uqP/3/hrKromV/Lg5/BmJUKdap6NRIqitA/LQbyMTHkw6GWARtx2A3THL8cCgEEJXGezq7yV7ZjeOkQkzD/CUmboaNDE25sg6Rfd7gbFLcHgofnvovM5woXGrvhPZsd+DGPqrc6EJYPRUEGYRRcpBpH8tk8oiUqTgarbAAY7Nf3CGwvucNPEl9XFURu2OiiVSc58eV5o6b1S7N/RyXYARb5aEbwvJp0nvtw+FFw8= 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: Hi Hugh, On 2025/7/16 16:08, Hugh Dickins wrote: > If swap_writeout() returns AOP_WRITEPAGE_ACTIVATE (for example, because > zswap cannot compress and memcg disables writeback), there is no virtue > in keeping that folio in swap cache and holding the swap allocation: > shmem_writeout() switch it back to shmem page cache before returning. > > Folio lock is held, and folio->memcg_data remains set throughout, so > there is no need to get into any memcg or memsw charge complications: > swap_free_nr() and delete_from_swap_cache() do as much as is needed (but > beware the race with shmem_free_swap() when inode truncated or evicted). > > Doing the same for an anonymous folio is harder, since it will usually > have been unmapped, with references to the swap left in the page tables. > Adding a function to remap the folio would be fun, but not worthwhile > unless it has other uses, or an urgent bug with anon is demonstrated. > > Signed-off-by: Hugh Dickins > --- > mm/shmem.c | 33 ++++++++++++++++++++++++++++++++- > 1 file changed, 32 insertions(+), 1 deletion(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 33675361031b..5a7ce4c8bad6 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -1655,6 +1655,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, > > if (!folio_alloc_swap(folio, __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN)) { > bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages); > + int error; > > /* > * Add inode to shmem_unuse()'s list of swapped-out inodes, > @@ -1675,7 +1676,37 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, > shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap)); > > BUG_ON(folio_mapped(folio)); > - return swap_writeout(folio, plug); > + error = swap_writeout(folio, plug); > + if (error != AOP_WRITEPAGE_ACTIVATE) { > + /* folio has been unlocked */ > + return error; > + } > + > + /* > + * The intention here is to avoid holding on to the swap when > + * zswap was unable to compress and unable to writeback; but > + * it will be appropriate if other reactivate cases are added. > + */ > + error = shmem_add_to_page_cache(folio, mapping, index, > + swp_to_radix_entry(folio->swap), > + __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN); > + /* Swap entry might be erased by racing shmem_free_swap() */ > + if (!error) { > + spin_lock(&info->lock); > + info->swapped -= nr_pages; > + spin_unlock(&info->lock); Using the helper 'shmem_recalc_inode(inode, 0, -nr_pages)' seems more readable? > + swap_free_nr(folio->swap, nr_pages); > + } > + > + /* > + * The delete_from_swap_cache() below could be left for > + * shrink_folio_list()'s folio_free_swap() to dispose of; > + * but I'm a little nervous about letting this folio out of > + * shmem_writeout() in a hybrid half-tmpfs-half-swap state > + * e.g. folio_mapping(folio) might give an unexpected answer. > + */ > + delete_from_swap_cache(folio); IIUC, Should the delete_from_swap_cache() also be moved into the 'if (!error)' branch? Since if shmem_free_swap() has freed the swap entry, it would also reclaim the swap cache, no?