| Message ID | 1583248190-18123-1-git-send-email-xianting_tian@126.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/filemap.c: clear page error before actual read | expand |
On Tue, Mar 03, 2020 at 10:09:50AM -0500, Xianting Tian wrote: > This patch is to add the calling of ClearPageError just before the > actual read of read path of cramfs mount. Thank you for your patch and your analysis. I concur; an error set on a page will not be cleared by doing a subsequent read. This is probably an oversight; in generic_file_buffered_read(), we do: ClearPageError(page); /* Start the actual read. The read will unlock the page. */ error = mapping->a_ops->readpage(filp, page); and also in filemap_fault: ClearPageError(page); fpin = maybe_unlock_mmap_for_io(vmf, fpin); error = mapping->a_ops->readpage(file, page); That said, a freshly allocated page will not have PageError set on it, so I would prefer to see this: /* Someone else locked and filled the page in a very small window */ if (PageUptodate(page)) { unlock_page(page); goto out; } + ClearPageError(page); goto filler;
thanks for your reply:) totally agree, it is more accurate as below, I will send the patch again, thanks. >+ ClearPageError(page); > goto filler; At 2020-03-04 00:43:17, "Matthew Wilcox" <willy@infradead.org> wrote: >On Tue, Mar 03, 2020 at 10:09:50AM -0500, Xianting Tian wrote: >> This patch is to add the calling of ClearPageError just before the >> actual read of read path of cramfs mount. > >Thank you for your patch and your analysis. I concur; an error set >on a page will not be cleared by doing a subsequent read. This is >probably an oversight; in generic_file_buffered_read(), we do: > > ClearPageError(page); > /* Start the actual read. The read will unlock the page. */ > error = mapping->a_ops->readpage(filp, page); > >and also in filemap_fault: > > ClearPageError(page); > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > error = mapping->a_ops->readpage(file, page); > >That said, a freshly allocated page will not have PageError set on >it, so I would prefer to see this: > > /* Someone else locked and filled the page in a very small window */ > if (PageUptodate(page)) { > unlock_page(page); > goto out; > } >+ ClearPageError(page); > goto filler;
hi Wilcox thanks for your reply:) totally agree, it is more accurate as below, I will send the patch again, thanks. + ClearPageError(page); goto filler; On 03/04/2020 00:43, Matthew Wilcox wrote: On Tue, Mar 03, 2020 at 10:09:50AM -0500, Xianting Tian wrote: > This patch is to add the calling of ClearPageError just before the > actual read of read path of cramfs mount. Thank you for your patch and your analysis. I concur; an error set on a page will not be cleared by doing a subsequent read. This is probably an oversight; in generic_file_buffered_read(), we do: ClearPageError(page); /* Start the actual read. The read will unlock the page. */ error = mapping->a_ops->readpage(filp, page); and also in filemap_fault: ClearPageError(page); fpin = maybe_unlock_mmap_for_io(vmf, fpin); error = mapping->a_ops->readpage(file, page); That said, a freshly allocated page will not have PageError set on it, so I would prefer to see this: /* Someone else locked and filled the page in a very small window */ if (PageUptodate(page)) { unlock_page(page); goto out; } + ClearPageError(page); goto filler;
diff --git a/mm/filemap.c b/mm/filemap.c index 1784478..d65428f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2755,6 +2755,13 @@ static struct page *do_read_cache_page(struct address_space *mapping, } filler: + /* + * A previous I/O error may have been due to temporary + * failures. + * Clear page error before actual read, PG_error will be + * set again if read page fails. + */ + ClearPageError(page); if (filler) err = filler(data, page); else
Mount failure issue happens under the scenario: Application forked dozens of threads to mount the same number of cramfs images separately in docker, but several mounts failed with high probability. Mount failed due to the checking result of the page(read from the superblock of loop dev) is not uptodate after wait_on_page_locked(page) returned in function cramfs_read: wait_on_page_locked(page); if (!PageUptodate(page)) { ... } The reason of the checking result of the page not uptodate: systemd-udevd read the loopX dev before mount, because the status of loopX is Lo_unbound at this time, so loop_make_request directly trigger the calling of io_end handler end_buffer_async_read, which called SetPageError(page). So It caused the page can't be set to uptodate in function end_buffer_async_read: if(page_uptodate && !PageError(page)) { SetPageUptodate(page); } Then mount operation is performed, it used the same page which is just accessed by systemd-udevd above, Because this page is not uptodate, it will launch a actual read via submit_bh, then wait on this page by calling wait_on_page_locked(page). When the I/O of the page done, io_end handler end_buffer_async_read is called, because no one cleared the page error(during the whole read path of mount), which is caused by systemd-udevd reading, so this page is still in "PageError" status, which can't be set to uptodate in function end_buffer_async_read, then caused mount failure. But sometimes mount succeed even through systemd-udeved read loopX dev just before, The reason is systemd-udevd launched other loopX read just between step 3.1 and 3.2, the steps as below: 1, loopX dev default status is Lo_unbound; 2, systemd-udved read loopX dev (page is set to PageError); 3, mount operation 1) set loopX status to Lo_bound; ==>systemd-udevd read loopX dev<== 2) read loopX dev(page has no error) 3) mount succeed As the loopX dev status is set to Lo_bound after step 3.1, so the other loopX dev read by systemd-udevd will go through the whole I/O stack, part of the call trace as below: SYS_read vfs_read do_sync_read blkdev_aio_read generic_file_aio_read do_generic_file_read: ClearPageError(page); mapping->a_ops->readpage(filp, page); here, mapping->a_ops->readpage() is blkdev_readpage. In latest kernel, some function name changed, the call trace as below: blkdev_read_iter generic_file_read_iter generic_file_buffered_read: /* * A previous I/O error may have been due to temporary * failures, eg. mutipath errors. * Pg_error will be set again if readpage fails. */ ClearPageError(page); /* Start the actual read. The read will unlock the page*/ error=mapping->a_ops->readpage(flip, page); We can see ClearPageError(page) is called before the actual read, then the read in step 3.2 succeed. This patch is to add the calling of ClearPageError just before the actual read of read path of cramfs mount. Without the patch, the call trace as below when performing cramfs mount: do_mount cramfs_read cramfs_blkdev_read read_cache_page do_read_cache_page: filler(data, page); or mapping->a_ops->readpage(data, page); With the patch, the call trace as below when performing mount: do_mount cramfs_read cramfs_blkdev_read read_cache_page: do_read_cache_page: ClearPageError(page); <== new add filler(data, page); or mapping->a_ops->readpage(data, page); With the patch, mount operation trigger the calling of ClearPageError(page) before the actual read, the page has no error if no additional page error happen when I/O done. Signed-off-by: Xianting Tian <xianting_tian@126.com> --- mm/filemap.c | 7 +++++++ 1 file changed, 7 insertions(+)