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Registered in England and Wales under Company Registration No. 3798903 Subject: [PATCH 1/2] nfs: Fix write to swapfile failure due to generic_write_checks() From: David Howells To: willy@infradead.org Cc: dhowells@redhat.com, trond.myklebust@primarydata.com, darrick.wong@oracle.com, hch@lst.de, jlayton@kernel.org, sfrench@samba.org, torvalds@linux-foundation.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Date: Thu, 12 Aug 2021 12:57:50 +0100 Message-ID: <162876946993.3068428.11259546375949085551.stgit@warthog.procyon.org.uk> In-Reply-To: <162876946134.3068428.15475611190876694695.stgit@warthog.procyon.org.uk> References: <162876946134.3068428.15475611190876694695.stgit@warthog.procyon.org.uk> User-Agent: StGit/0.23 MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=DhaQlQ1V; spf=none (imf06.hostedemail.com: domain of dhowells@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=dhowells@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 604F9802445D X-Stat-Signature: tjr87f4k66op4guzacsu6p3u3batwn9k X-HE-Tag: 1628769476-44528 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: Trying to use a swapfile on NFS results in every DIO write failing with ETXTBSY because generic_write_checks(), as called by nfs_direct_write() from nfs_direct_IO(), forbids writes to swapfiles. Fix this by introducing a new kiocb flag, IOCB_SWAP, that's set by the swap code to indicate that the swapper is doing this operation and so overrule the check in generic_write_checks(). Without this patch, the following is seen: Write error on dio swapfile (3800334336) Altering __swap_writepage() to show the error shows: Write error (-26) on dio swapfile (3800334336) Tested by swapping off all swap partitions and then swapping on a prepared NFS file (CONFIG_NFS_SWAP=y is also needed). Enough copies of the following program then need to be run to force swapping to occur (at least one per gigabyte of RAM): #include #include #include #include #include int main() { unsigned int pid = getpid(), iterations = 0; size_t i, j, size = 1024 * 1024 * 1024; char *p; bool mismatch; p = malloc(size); if (!p) { perror("malloc"); exit(1); } srand(pid); for (i = 0; i < size; i += 4) *(unsigned int *)(p + i) = rand(); do { for (j = 0; j < 16; j++) { for (i = 0; i < size; i += 4096) *(unsigned int *)(p + i) += 1; iterations++; } mismatch = false; srand(pid); for (i = 0; i < size; i += 4) { unsigned int r = rand(); unsigned int v = *(unsigned int *)(p + i); if (i % 4096 == 0) v -= iterations; if (v != r) { fprintf(stderr, "mismatch %zx: %x != %x (diff %x)\n", i, v, r, v - r); mismatch = true; } } } while (!mismatch); exit(1); } Fixes: dc617f29dbe5 ("vfs: don't allow writes to swap files") Signed-off-by: David Howells cc: Darrick J. Wong cc: Christoph Hellwig cc: Trond Myklebust cc: linux-nfs@vger.kernel.org --- fs/read_write.c | 2 +- include/linux/fs.h | 1 + mm/page_io.c | 7 ++++--- 3 files changed, 6 insertions(+), 4 deletions(-) diff --git a/fs/read_write.c b/fs/read_write.c index 9db7adf160d2..daef721ca67e 100644 --- a/fs/read_write.c +++ b/fs/read_write.c @@ -1646,7 +1646,7 @@ ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from) loff_t count; int ret; - if (IS_SWAPFILE(inode)) + if (IS_SWAPFILE(inode) && !(iocb->ki_flags & IOCB_SWAP)) return -ETXTBSY; if (!iov_iter_count(from)) diff --git a/include/linux/fs.h b/include/linux/fs.h index 640574294216..b3e6a20f28ef 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -319,6 +319,7 @@ enum rw_hint { /* iocb->ki_waitq is valid */ #define IOCB_WAITQ (1 << 19) #define IOCB_NOIO (1 << 20) +#define IOCB_SWAP (1 << 21) /* This is a swap request */ struct kiocb { struct file *ki_filp; diff --git a/mm/page_io.c b/mm/page_io.c index d597bc6e6e45..edb72bf624d2 100644 --- a/mm/page_io.c +++ b/mm/page_io.c @@ -303,7 +303,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc, iov_iter_bvec(&from, WRITE, &bv, 1, PAGE_SIZE); init_sync_kiocb(&kiocb, swap_file); - kiocb.ki_pos = page_file_offset(page); + kiocb.ki_pos = page_file_offset(page); + kiocb.ki_flags = IOCB_DIRECT | IOCB_WRITE | IOCB_SWAP; set_page_writeback(page); unlock_page(page); @@ -324,8 +325,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc, */ set_page_dirty(page); ClearPageReclaim(page); - pr_err_ratelimited("Write error on dio swapfile (%llu)\n", - page_file_offset(page)); + pr_err_ratelimited("Write error (%d) on dio swapfile (%llu)\n", + ret, page_file_offset(page)); } end_page_writeback(page); return ret;