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Date:   Tue, 18 Feb 2020 19:35:34 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     cluster-devel@...hat.com, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-mm@...ck.org, linux-btrfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, ocfs2-devel@....oracle.com
Subject: Re: [PATCH v6 08/19] mm: Add readahead address space operation

On Tue, Feb 18, 2020 at 07:10:44PM -0800, Eric Biggers wrote:
> On Mon, Feb 17, 2020 at 10:45:54AM -0800, Matthew Wilcox wrote:
> > diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
> > index 7d4d09dd5e6d..81ab30fbe45c 100644
> > --- a/Documentation/filesystems/vfs.rst
> > +++ b/Documentation/filesystems/vfs.rst
> > @@ -706,6 +706,7 @@ cache in your filesystem.  The following members are defined:
> >  		int (*readpage)(struct file *, struct page *);
> >  		int (*writepages)(struct address_space *, struct writeback_control *);
> >  		int (*set_page_dirty)(struct page *page);
> > +		void (*readahead)(struct readahead_control *);
> >  		int (*readpages)(struct file *filp, struct address_space *mapping,
> >  				 struct list_head *pages, unsigned nr_pages);
> >  		int (*write_begin)(struct file *, struct address_space *mapping,
> > @@ -781,12 +782,24 @@ cache in your filesystem.  The following members are defined:
> >  	If defined, it should set the PageDirty flag, and the
> >  	PAGECACHE_TAG_DIRTY tag in the radix tree.
> >  
> > +``readahead``
> > +	Called by the VM to read pages associated with the address_space
> > +	object.  The pages are consecutive in the page cache and are
> > +	locked.  The implementation should decrement the page refcount
> > +	after starting I/O on each page.  Usually the page will be
> > +	unlocked by the I/O completion handler.  If the function does
> > +	not attempt I/O on some pages, the caller will decrement the page
> > +	refcount and unlock the pages for you.	Set PageUptodate if the
> > +	I/O completes successfully.  Setting PageError on any page will
> > +	be ignored; simply unlock the page if an I/O error occurs.
> > +
> 
> This is unclear about how "not attempting I/O" works and how that affects who is
> responsible for putting and unlocking the pages.  How does the caller know which
> pages were not attempted?  Can any arbitrary subset of pages be not attempted,
> or just the last N pages?
> 
> In the code, the caller actually uses readahead_for_each() to iterate through
> and put+unlock the pages.  That implies that ->readahead() must also use
> readahead_for_each() as well, in order for the iterator to be advanced
> correctly... Right?  And the ownership of each page is transferred to the callee
> when the callee advances the iterator past that page.
> 
> I don't see how ext4_readahead() and f2fs_readahead() can work at all, given
> that they don't advance the iterator.
> 

Yep, this patchset immediately crashes on boot with:

BUG: Bad page state in process swapper/0  pfn:02176
page:ffffea00000751d0 refcount:0 mapcount:0 mapping:ffff88807cba0400 index:0x0
ext4_da_aops name:"systemd"
flags: 0x100000000020001(locked|mappedtodisk)
raw: 0100000000020001 dead000000000100 dead000000000122 ffff88807cba0400
raw: 0000000000000000 0000000000000000 00000000ffffffff
page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
bad because of flags: 0x1(locked)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc2-00019-g7203ed9018cb9 #18
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x7a/0xaa lib/dump_stack.c:118
 bad_page.cold+0x89/0xba mm/page_alloc.c:649
 free_pages_check_bad+0x5d/0x60 mm/page_alloc.c:1050
 free_pages_check mm/page_alloc.c:1059 [inline]
 free_pages_prepare mm/page_alloc.c:1157 [inline]
 free_pcp_prepare+0x1c1/0x200 mm/page_alloc.c:1198
 free_unref_page_prepare mm/page_alloc.c:3011 [inline]
 free_unref_page+0x16/0x70 mm/page_alloc.c:3060
 __put_single_page mm/swap.c:81 [inline]
 __put_page+0x31/0x40 mm/swap.c:115
 put_page include/linux/mm.h:1029 [inline]
 ext4_mpage_readpages+0x778/0x9b0 fs/ext4/readpage.c:405
 ext4_readahead+0x2f/0x40 fs/ext4/inode.c:3242
 read_pages+0x4c/0x200 mm/readahead.c:126
 page_cache_readahead_limit+0x224/0x250 mm/readahead.c:241
 __do_page_cache_readahead mm/readahead.c:266 [inline]
 ra_submit mm/internal.h:62 [inline]
 ondemand_readahead+0x1df/0x4d0 mm/readahead.c:544
 page_cache_sync_readahead+0x2d/0x40 mm/readahead.c:579
 generic_file_buffered_read+0x77e/0xa90 mm/filemap.c:2029
 generic_file_read_iter+0xd4/0x130 mm/filemap.c:2302
 ext4_file_read_iter fs/ext4/file.c:131 [inline]
 ext4_file_read_iter+0x53/0x180 fs/ext4/file.c:114
 call_read_iter include/linux/fs.h:1897 [inline]
 new_sync_read+0x113/0x1a0 fs/read_write.c:414
 __vfs_read+0x21/0x30 fs/read_write.c:427
 vfs_read+0xcb/0x160 fs/read_write.c:461
 kernel_read+0x2c/0x40 fs/read_write.c:440
 prepare_binprm+0x14f/0x190 fs/exec.c:1589
 __do_execve_file.isra.0+0x4c0/0x800 fs/exec.c:1806
 do_execveat_common fs/exec.c:1871 [inline]
 do_execve+0x20/0x30 fs/exec.c:1888
 run_init_process+0xc8/0xcd init/main.c:1279
 try_to_run_init_process+0x10/0x36 init/main.c:1288
 kernel_init+0xac/0xfd init/main.c:1385
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Disabling lock debugging due to kernel taint
page:ffffea00000751d0 refcount:0 mapcount:0 mapping:ffff88807cba0400 index:0x0
ext4_da_aops name:"systemd"
flags: 0x100000000020001(locked|mappedtodisk)
raw: 0100000000020001 dead000000000100 dead000000000122 ffff88807cba0400
raw: 0000000000000000 0000000000000000 00000000ffffffff
page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)


I had to add:

diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index e14841ade6122..cb982088b5225 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -401,8 +401,10 @@ int ext4_mpage_readpages(struct address_space *mapping,
 		else
 			unlock_page(page);
 	next_page:
-		if (rac)
+		if (rac) {
 			put_page(page);
+			readahead_next(rac);
+		}
 	}
 	if (bio)
 		submit_bio(bio);
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 87964e4cb6b81..e16b0fe42e2e5 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2238,8 +2238,10 @@ int f2fs_mpage_readpages(struct inode *inode, struct readahead_control *rac,
 #ifdef CONFIG_F2FS_FS_COMPRESSION
 next_page:
 #endif
-		if (rac)
+		if (rac) {
 			put_page(page);
+			readahead_next(rac);
+		}
 
 #ifdef CONFIG_F2FS_FS_COMPRESSION
 		if (f2fs_compressed_file(inode)) {


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