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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id k2-20020a37ba02000000b0067dc1b0104asm13964961qkf.124.2022.04.08.13.38.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 Apr 2022 13:38:53 -0700 (PDT) Date: Fri, 8 Apr 2022 13:38:41 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Andrew Morton cc: Hugh Dickins , Chuck Lever III , Mark Hemment , Patrice CHOTARD , Mikulas Patocka , Lukas Czerner , Christoph Hellwig , "Darrick J. Wong" , linux-mm@kvack.org, linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com Subject: [PATCH] tmpfs: fix regressions from wider use of ZERO_PAGE Message-ID: <9a978571-8648-e830-5735-1f4748ce2e30@google.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org Chuck Lever reported fsx-based xfstests generic 075 091 112 127 failing when 5.18-rc1 NFS server exports tmpfs: bisected to recent tmpfs change. Whilst nfsd_splice_action() does contain some questionable handling of repeated pages, and Chuck was able to work around there, history from Mark Hemment makes clear that there might be similar dangers elsewhere: it was not a good idea for me to pass ZERO_PAGE down to unknown actors. Revert shmem_file_read_iter() to using ZERO_PAGE for holes only when iter_is_iovec(); in other cases, use the more natural iov_iter_zero() instead of copy_page_to_iter(). We would use iov_iter_zero() throughout, but the x86 clear_user() is not nearly so well optimized as copy to user (dd of 1T sparse tmpfs file takes 57 seconds rather than 44 seconds). And now pagecache_init() does not need to SetPageUptodate(ZERO_PAGE(0)): which had caused boot failure on arm noMMU STM32F7 and STM32H7 boards Reported-by: Patrice CHOTARD Reported-by: Chuck Lever III Fixes: 56a8c8eb1eaf ("tmpfs: do not allocate pages on read") Signed-off-by: Hugh Dickins Tested-by: Chuck Lever III --- mm/filemap.c | 6 ------ mm/shmem.c | 31 ++++++++++++++++++++----------- 2 files changed, 20 insertions(+), 17 deletions(-) --- 5.18-rc1/mm/filemap.c +++ linux/mm/filemap.c @@ -1063,12 +1063,6 @@ void __init pagecache_init(void) init_waitqueue_head(&folio_wait_table[i]); page_writeback_init(); - - /* - * tmpfs uses the ZERO_PAGE for reading holes: it is up-to-date, - * and splice's page_cache_pipe_buf_confirm() needs to see that. - */ - SetPageUptodate(ZERO_PAGE(0)); } /* --- 5.18-rc1/mm/shmem.c +++ linux/mm/shmem.c @@ -2513,7 +2513,6 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) pgoff_t end_index; unsigned long nr, ret; loff_t i_size = i_size_read(inode); - bool got_page; end_index = i_size >> PAGE_SHIFT; if (index > end_index) @@ -2570,24 +2569,34 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) */ if (!offset) mark_page_accessed(page); - got_page = true; + /* + * Ok, we have the page, and it's up-to-date, so + * now we can copy it to user space... + */ + ret = copy_page_to_iter(page, offset, nr, to); + put_page(page); + + } else if (iter_is_iovec(to)) { + /* + * Copy to user tends to be so well optimized, but + * clear_user() not so much, that it is noticeably + * faster to copy the zero page instead of clearing. + */ + ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to); } else { - page = ZERO_PAGE(0); - got_page = false; + /* + * But submitting the same page twice in a row to + * splice() - or others? - can result in confusion: + * so don't attempt that optimization on pipes etc. + */ + ret = iov_iter_zero(nr, to); } - /* - * Ok, we have the page, and it's up-to-date, so - * now we can copy it to user space... - */ - ret = copy_page_to_iter(page, offset, nr, to); retval += ret; offset += ret; index += offset >> PAGE_SHIFT; offset &= ~PAGE_MASK; - if (got_page) - put_page(page); if (!iov_iter_count(to)) break; if (ret < nr) {