Message-ID: <20250729202151.GD2672049@frogsfrogsfrogs>
Date: Tue, 29 Jul 2025 13:21:51 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Joanne Koong <joannelkoong@...il.com>
Cc: Naresh Kamboju <naresh.kamboju@...aro.org>,
	linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	linux-xfs@...r.kernel.org, open list <linux-kernel@...r.kernel.org>,
	lkft-triage@...ts.linaro.org,
	Linux Regressions <regressions@...ts.linux.dev>,
	Miklos Szeredi <miklos@...redi.hu>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <liam.howlett@...cle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Anders Roxell <anders.roxell@...aro.org>,
	Ben Copeland <benjamin.copeland@...aro.org>
Subject: Re: next-20250721 arm64 16K and 64K page size WARNING fs fuse file.c
 at fuse_iomap_writeback_range

On Mon, Jul 28, 2025 at 02:28:31PM -0700, Joanne Koong wrote:
> On Mon, Jul 28, 2025 at 12:11 PM Darrick J. Wong <djwong@...nel.org> wrote:
> >
> > On Mon, Jul 28, 2025 at 10:44:01AM -0700, Joanne Koong wrote:
> > > On Mon, Jul 28, 2025 at 10:14 AM Darrick J. Wong <djwong@...nel.org> wrote:
> > > >
> > > > On Fri, Jul 25, 2025 at 06:16:15PM -0700, Joanne Koong wrote:
> > > > > On Thu, Jul 24, 2025 at 12:14 PM Joanne Koong <joannelkoong@...il.com> wrote:
> > > > > >
> > > > > > On Wed, Jul 23, 2025 at 3:37 PM Joanne Koong <joannelkoong@...il.com> wrote:
> > > > > > >
> > > > > > > On Wed, Jul 23, 2025 at 2:20 PM Darrick J. Wong <djwong@...nel.org> wrote:
> > > > > > > >
> > > > > > > > On Wed, Jul 23, 2025 at 11:42:42AM -0700, Joanne Koong wrote:
> > > > > > > > > On Wed, Jul 23, 2025 at 7:46 AM Darrick J. Wong <djwong@...nel.org> wrote:
> > > > > > > > > >
> > > > > > > > > > [cc Joanne]
> > > > > > > > > >
> > > > > > > > > > On Wed, Jul 23, 2025 at 05:14:28PM +0530, Naresh Kamboju wrote:
> > > > > > > > > > > Test regression: next-20250721 arm64 16K and 64K page size WARNING fs
> > > > > > > > > > > fuse file.c at fuse_iomap_writeback_range
> > > > > > > > > > >
> > > > > > > > > > > Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
> > > > > > > > > > >
> > > > > > > > > > > ## Test log
> > > > > > > > > > > ------------[ cut here ]------------
> > > > > > > > > > > [  343.828105] WARNING: fs/fuse/file.c:2146 at
> > > > > > > > > > > fuse_iomap_writeback_range+0x478/0x558 [fuse], CPU#0: msync04/4190
> > > > > > > > > >
> > > > > > > > > >         WARN_ON_ONCE(len & (PAGE_SIZE - 1));
> > > > > > > > > >
> > > > > > > > > > /me speculates that this might be triggered by an attempt to write back
> > > > > > > > > > some 4k fsblock within a 16/64k base page?
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > > I think this can happen on 4k base pages as well actually. On the
> > > > > > > > > iomap side, the length passed is always block-aligned and in fuse, we
> > > > > > > > > set blkbits to be PAGE_SHIFT so theoretically block-aligned is always
> > > > > > > > > page-aligned, but I missed that if it's a "fuseblk" filesystem, that
> > > > > > > > > isn't true and the blocksize is initialized to a default size of 512
> > > > > > > > > or whatever block size is passed in when it's mounted.
> > > > > > > >
> > > > > > > > <nod> I think you're correct.
> > > > > > > >
> > > > > > > > > I'll send out a patch to remove this line. It doesn't make any
> > > > > > > > > difference for fuse_iomap_writeback_range() logic whether len is
> > > > > > > > > page-aligned or not; I had added it as a sanity-check against sketchy
> > > > > > > > > ranges.
> > > > > > > > >
> > > > > > > > > Also, I just noticed that apparently the blocksize can change
> > > > > > > > > dynamically for an inode in fuse through getattr replies from the
> > > > > > > > > server (see fuse_change_attributes_common()). This is a problem since
> > > > > > > > > the iomap uses inode->i_blkbits for reading/writing to the bitmap. I
> > > > > > > > > think we will have to cache the inode blkbits in the iomap_folio_state
> > > > > > > > > struct unfortunately :( I'll think about this some more and send out a
> > > > > > > > > patch for this.
> > > > > > > >
> > > > > > > > From my understanding of the iomap code, it's possible to do that if you
> > > > > > > > flush and unmap the entire pagecache (whilst holding i_rwsem and
> > > > > > > > mmap_invalidate_lock) before you change i_blkbits.  Nobody *does* this
> > > > > > > > so I have no idea if it actually works, however.  Note that even I don't
> > > > > > > > implement the flush and unmap bit; I just scream loudly and do nothing:
> > > > > > >
> > > > > > > lol! i wish I could scream loudly and do nothing too for my case.
> > > > > > >
> > > > > > > AFAICT, I think I just need to flush and unmap that file and can leave
> > > > > > > the rest of the files/folios in the pagecache as is? But then if the
> > > > > > > file has active refcounts on it or has been pinned into memory, can I
> > > > > > > still unmap and remove it from the page cache? I see the
> > > > > > > invalidate_inode_pages2() function but my understanding is that the
> > > > > > > page still stays in the cache if it has has active references, and if
> > > > > > > the page gets mmaped and there's a page fault on it, it'll end up
> > > > > > > using the preexisting old page in the page cache.
> > > > > >
> > > > > > Never mind, I was mistaken about this. Johannes confirmed that even if
> > > > > > there's active refcounts on the folio, it'll still get removed from
> > > > > > the page cache after unmapping and the page cache reference will get
> > > > > > dropped.
> > > > > >
> > > > > > I think I can just do what you suggested and call
> > > > > > filemap_invalidate_inode() in fuse_change_attributes_common() then if
> > > > > > the inode blksize gets changed. Thanks for the suggestion!
> > > > > >
> > > > >
> > > > > Thinking about this some more, I don't think this works after all
> > > > > because the writeback + page cache removal and inode blkbits update
> > > > > needs to be atomic, else after we write back and remove the pages from
> > > > > the page cache, a write could be issued right before we update the
> > > > > inode blkbits. I don't think we can hold the inode lock the whole time
> > > > > for it either since writeback could be intensive. (also btw, I
> > > > > realized in hindsight that invalidate_inode_pages2_range() would have
> > > > > been the better function to call instead of
> > > > > filemap_invalidate_inode()).
> > > > >
> > > > > > >
> > > > > > > I don't think I really need to have it removed from the page cache so
> > > > > > > much as just have the ifs state for all the folios in the file freed
> > > > > > > (after flushing the file) so that it can start over with a new ifs.
> > > > > > > Ideally we could just flush the file, then iterate through all the
> > > > > > > folios in the mapping in order of ascending index, and kfree their
> > > > > > > ->private, but I'm not seeing how we can prevent the case of new
> > > > > > > writes / a new ifs getting allocated for folios at previous indexes
> > > > > > > while we're trying to do the iteration/kfreeing.
> > > > > > >
> > > > >
> > > > > Going back to this idea, I think this can work. I realized we don't
> > > > > need to flush the file, it's enough to free the ifs, then update the
> > > > > inode->i_blkbits, then reallocate the ifs (which will now use the
> > > > > updated blkbits size), and if we hold the inode lock throughout, that
> > > > > prevents any concurrent writes.
> > > > > Something like:
> > > > >      inode_lock(inode);
> > > > >      XA_STATE(xas, &mapping->i_pages, 0);
> > > > >      xa_lock_irq(&mapping->i_pages);
> > > > >      xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
> > > > >           folio_lock(folio);
> > > > >           if (folio_test_dirty(folio)) {
> > > > >                   folio_wait_writeback(folio);
> > > > >                   kfree(folio->private);
> > > > >           }
> >
> > Heh, I didn't even see this chunk, distracted as I am today. :/
> >
> > So this doesn't actually /initiate/ writeback, it just waits
> > (potentially for a long time) for someone else to come along and do it.
> > That might not be what you want since the blocksize change will appear
> > to stall while nothing else is going on in the system.
> 
> I thought if the folio isn't under writeback then
> folio_wait_writeback() just returns immediately as a no-op.
> I don't think we need/want to initiate writeback, I think we only need
> to ensure that if it is already under writeback, that writeback
> finishes while it uses the old i_blksize so nothing gets corrupted. As
> I understand it (but maybe I'm misjudging this), holding the inode
> lock and then initiating writeback is too much given that writeback
> can take a long time (eg if the fuse server writes the data over some
> network).
> 
> >
> > Also, unless you're going to put this in buffered-io.c, it's not
> > desirable for a piece of code to free something it didn't allocate.
> > IOWs, I don't think it's a good idea for *fuse* to go messing with a
> > folio->private that iomap set.
> 
> Okay, good point. I agree. I was hoping to have this not bleed into
> the iomap library but maybe there's no getting around that in a good
> way.

<shrug> Any other filesystem that has mutable file block size is going
to need something to enact a change.

> >
> > > > >           folio_unlock(folio);
> > > > >      }
> > > > >     inode->i_blkbits = new_blkbits_size;
> > > >
> > > > The trouble is, you also have to resize the iomap_folio_state objects
> > > > attached to each folio if you change i_blkbits...
> > >
> > > I think the iomap_folio_state objects automatically get resized here,
> > > no? We first kfree the folio->private which kfrees the entire ifs,
> >
> > Err, right, it does free the ifs and recreate it later if necessary.
> >
> > > then we change inode->i_blkbits to the new size, then when we call
> > > folio_mark_dirty(), it'll create the new ifs which creates a new folio
> > > state object using the new/updated i_blkbits size
> > >
> > > >
> > > > >     xas_set(&xas, 0);
> > > > >     xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
> > > > >           folio_lock(folio);
> > > > >           if (folio_test_dirty(folio) && !folio_test_writeback(folio))
> > > > >                  folio_mark_dirty(folio);
> > > >
> > > > ...because iomap_dirty_folio doesn't know how to reallocate the folio
> > > > state object in response to i_blkbits having changed.
> >
> > Also, what about clean folios that have an ifs?  You'd still need to
> > handle the ifs's attached to those.
> 
> Ah you're right, there could be clean folios there too that have an
> ifs. I think in the above logic, if we iterate through all
> mapping->i_pages (not just PAGECACHE_TAG_DIRTY marked ones) and move
> the kfree to after the "if (folio_test_dirty(folio))" block, then it
> addresses that case. eg something like this:
> 
>      inode_lock(inode);
>      XA_STATE(xas, &mapping->i_pages, 0);
>      xa_lock_irq(&mapping->i_pages);
>      xas_for_each(&xas, folio, ULONG_MAX) {
>           folio_lock(folio);
>           if (folio_test_dirty(folio))
>                   folio_wait_writeback(folio);
>           kfree(folio->private);
>           folio_unlock(folio);
>      }
>     inode->i_blkbits = new_blkbits;
>     xas_set(&xas, 0);
>     xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
>           folio_lock(folio);
>           if (folio_test_dirty(folio) && !folio_test_writeback(folio))
>                  folio_mark_dirty(folio);
>           folio_unlock(folio);
>     }
>     xa_unlock_irq(&mapping->i_pages);
>     inode_unlock(inode);
> 
> 
> >
> > So I guess if you wanted iomap to handle a blocksize change, you could
> > do something like:
> >
> > iomap_change_file_blocksize(inode, new_blkbits) {
> >         inode_lock()
> >         filemap_invalidate_lock()
> >
> >         inode_dio_wait()
> >         filemap_write_and_wait()
> >         if (new_blkbits > mapping_min_folio_order()) {
> >                 truncate_pagecache()
> >                 inode->i_blkbits = new_blkbits;
> >         } else {
> >                 inode->i_blkbits = new_blkbits;
> >                 xas_for_each(...) {
> >                         <create new ifs>
> >                         <translate uptodate/dirty state to new ifs>
> >                         <swap ifs>
> >                         <free old ifs>
> >                 }
> >         }
> >
> >         filemap_invalidate_unlock()
> >         inode_unlock()
> > }
> 
> Do you prefer this logic to the one above that walks through
> &mapping->i_pages? If so, then I'll go with this way.

Yes.  iomap should not be tightly bound to the pagecache's xarray; I
don't even really like the xas_for_each that I suggested above.

> The part I'm unsure about is that this logic seems more disruptive (eg
> initiating writeback while holding the inode lock and doing work for
> unmapping/page cache removal) than the other approach, but I guess
> this is also rare enough that it doesn't matter much.

I hope it's rare enough that doing truncate_pagecache unconditionally
won't be seen as a huge burden.

iomap_change_file_blocksize(inode, new_blkbits) {
        inode_dio_wait()
        filemap_write_and_wait()
        truncate_pagecache()

        inode->i_blkbits = new_blkbits;
}

fuse_file_change_blocksize(inode, new_blkbits) {
        inode_lock()
        filemap_invalidate_lock()

	iomap_change_file_blocksize(inode, new_blkbits);

        filemap_invalidate_unlock()
        inode_unlock()
}

Though my question remains -- is there a fuse filesystem that changes
the blocksize at runtime such that we can test this??

--D

> Thanks,
> Joanne
> 
> >
> > --D
> >
> > > > --D
> > > >
> > > > >           folio_unlock(folio);
> > > > >     }
> > > > >     xa_unlock_irq(&mapping->i_pages);
> > > > >     inode_unlock(inode);
> > > > >
> > > > >
> > > > > I think this is the only approach that doesn't require changes to iomap.
> > > > >
> > > > > I'm going to think about this some more next week and will try to send
> > > > > out a patch for this then.
> > > > >
> > > > >
> > > > > Thanks,
> > > > > Joanne
> > > > >
> > > > > > > >
> > > > > > > > void fuse_iomap_set_i_blkbits(struct inode *inode, u8 new_blkbits)
> > > > > > > > {
> > > > > > > >         trace_fuse_iomap_set_i_blkbits(inode, new_blkbits);
> > > > > > > >
> > > > > > > >         if (inode->i_blkbits == new_blkbits)
> > > > > > > >                 return;
> > > > > > > >
> > > > > > > >         if (!S_ISREG(inode->i_mode))
> > > > > > > >                 goto set_it;
> > > > > > > >
> > > > > > > >         /*
> > > > > > > >          * iomap attaches per-block state to each folio, so we cannot allow
> > > > > > > >          * the file block size to change if there's anything in the page cache.
> > > > > > > >          * In theory, fuse servers should never be doing this.
> > > > > > > >          */
> > > > > > > >         if (inode->i_mapping->nrpages > 0) {
> > > > > > > >                 WARN_ON(inode->i_blkbits != new_blkbits &&
> > > > > > > >                         inode->i_mapping->nrpages > 0);
> > > > > > > >                 return;
> > > > > > > >         }
> > > > > > > >
> > > > > > > > set_it:
> > > > > > > >         inode->i_blkbits = new_blkbits;
> > > > > > > > }
> > > > > > > >
> > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=fuse-iomap-attrs&id=da9b25d994c1140aae2f5ebf10e54d0872f5c884
> > > > > > > >
> > > > > > > > --D
> > > > > > > >
> > > > > > > > >
> > > > > > > > > Thanks,
> > > > > > > > > Joanne
> > > > > > > > >
> > > > >
> 

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