On Mon, Sep 20, 2021 at 3:35 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > On Mon, Sep 20, 2021 at 2:50 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Mon, Sep 20, 2021 at 02:23:41PM -0700, Yang Shi wrote: > > > On Sun, Sep 19, 2021 at 7:41 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > On Fri, Sep 17, 2021 at 05:07:03PM -0700, Yang Shi wrote: > > > > > > The debugging showed the page passed to invalidatepage is a huge page > > > > > > and the length is the size of huge page instead of single page due to > > > > > > read only FS THP support. But block_invalidatepage() would throw BUG if > > > > > > the size is greater than single page. > > > > > > > > Things have already gone wrong before we get to this point. See > > > > do_dentry_open(). You aren't supposed to be able to get a writable file > > > > descriptor on a file which has had huge pages added to the page cache > > > > without the filesystem's knowledge. That's the problem that needs to > > > > be fixed. > > > > > > I don't quite understand your point here. Do you mean do_dentry_open() > > > should fail for such cases instead of truncating the page cache? > > > > No, do_dentry_open() should have truncated the page cache when it was > > called and found that there were THPs in the cache. Then khugepaged > > should see that someone has the file open for write and decline to create > > new THPs. So it shouldn't be possible to get here with THPs in the cache. > I think Hugh's skipping special file patch (https://lore.kernel.org/linux-mm/a07564a3-b2fc-9ffe-3ace-3f276075ea5c@xxxxxxxxxx/) could fix this specific BUG report and seems like a more proper fix to this. However, it still doesn't make too much sense to have thp_size passed to do_invalidatepage(), then have PAGE_SIZE hardcoded in a BUG assertion IMHO. So it seems this patch is still useful because block_invalidatepage() is called by a few filesystems as well, for example, ext4. Or I'm wondering whether we should call do_invalidatepage() for each subpage of THP in truncate_cleanup_page() since private is for each subpage IIUC. > AFAICT, it does so. > > In do_dentry_open(): > /* > * XXX: Huge page cache doesn't support writing yet. Drop all page > * cache for this file before processing writes. > */ > if (f->f_mode & FMODE_WRITE) { > /* > * Paired with smp_mb() in collapse_file() to ensure nr_thps > * is up to date and the update to i_writecount by > * get_write_access() is visible. Ensures subsequent insertion > * of THPs into the page cache will fail. > */ > smp_mb(); > if (filemap_nr_thps(inode->i_mapping)) > truncate_pagecache(inode, 0); > } > > > In khugepaged: > filemap_nr_thps_inc(mapping); > /* > * Paired with smp_mb() in do_dentry_open() to ensure > * i_writecount is up to date and the update to nr_thps is > * visible. Ensures the page cache will be truncated if the > * file is opened writable. > */ > smp_mb(); > if (inode_is_open_for_write(mapping->host)) { > result = SCAN_FAIL; > __mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr); > filemap_nr_thps_dec(mapping); > goto xa_locked; > } > > But I'm not quite sure if there is any race condition.
