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Message-ID: <27pzaod6lj2mwn3refns5c4titbsrqhxcxjeuae6piom6rzqgy@mxb4m4jyej5s>
Date: Thu, 9 Jan 2025 15:38:33 +1100
From: Alistair Popple <apopple@...dia.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, lina@...hilina.net,
zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com, vishal.l.verma@...el.com,
dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com, jack@...e.cz,
jgg@...pe.ca, catalin.marinas@....com, will@...nel.org, mpe@...erman.id.au,
npiggin@...il.com, dave.hansen@...ux.intel.com, ira.weiny@...el.com,
willy@...radead.org, djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com,
david@...hat.com, peterx@...hat.com, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
jhubbard@...dia.com, hch@....de, david@...morbit.com, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v5 01/25] fuse: Fix dax truncate/punch_hole fault path
On Wed, Jan 08, 2025 at 02:30:24PM -0800, Dan Williams wrote:
> Alistair Popple wrote:
> > FS DAX requires file systems to call into the DAX layout prior to
> > unlinking inodes to ensure there is no ongoing DMA or other remote
> > access to the direct mapped page. The fuse file system implements
> > fuse_dax_break_layouts() to do this which includes a comment
> > indicating that passing dmap_end == 0 leads to unmapping of the whole
> > file.
> >
> > However this is not true - passing dmap_end == 0 will not unmap
> > anything before dmap_start, and further more
> > dax_layout_busy_page_range() will not scan any of the range to see if
> > there maybe ongoing DMA access to the range.
>
> It would be useful to clarify that this is bug was found by inspection
> and that there are no known end user reports of trouble but that the
> failure more would look like random fs corruption. The window is hard to
> hit because a block needs to be truncated, reallocated to
> a file, and written to before stale DMA could corrupt it. So that may
> contribute to the fact that fuse-dax users have not reported an issue
> since v5.10.
>
> > Fix this by checking for dmap_end == 0 in fuse_dax_break_layouts() and
> > pass the entire file range to dax_layout_busy_page_range().
>
> That's not what this patch does, maybe a rebase error that pushed the
> @dmap_end fixup after the call to dax_layout_busy_page_range?
Ha. Yep. I spotted this when doing the conversion to
dax_layout_busy_page_range() and then had to rebase it into a stand alone patch
for easy review. Obviously I put the check in the wrong spot, although it ends
up in the right spot at the end of the series.
> However, I don't think this is quite the right fix, more below...
Yeah, I like your version better so will respin with that.
> > Signed-off-by: Alistair Popple <apopple@...dia.com>
> > Fixes: 6ae330cad6ef ("virtiofs: serialize truncate/punch_hole and dax fault path")
> > Cc: Vivek Goyal <vgoyal@...hat.com>
> >
> > ---
> >
> > I am not at all familiar with the fuse file system driver so I have no
> > idea if the comment is relevant or not and whether the documented
> > behaviour for dmap_end == 0 is ever relied upon. However this seemed
> > like the safest fix unless someone more familiar with fuse can confirm
> > that dmap_end == 0 is never used.
>
> It is used in several places and has been wrong since day one. I believe
> the original commit simply misunderstood that
> dax_layout_busy_page_range() semantics are analogous to
> invalidate_inode_pages2_range() semantics in terms of what @start and
> @end mean.
>
> You can add:
>
> Co-developed-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> ...if you end up doing a resend, or I will add it on applying to
> nvdimm.git if the rebase does not end up being too prickly.
Looks like I accidentally dropped a PPC fix so will do a respin for that. And
the kernel build bot was complaining about incorrect documentation so will fix
that while I'm at it.
> -- 8< --
> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
> index c5d1feaa239c..455c4a16080b 100644
> --- a/fs/fuse/dax.c
> +++ b/fs/fuse/dax.c
> @@ -681,7 +681,6 @@ static int __fuse_dax_break_layouts(struct inode *inode, bool *retry,
> 0, 0, fuse_wait_dax_page(inode));
> }
>
> -/* dmap_end == 0 leads to unmapping of whole file */
> int fuse_dax_break_layouts(struct inode *inode, u64 dmap_start,
> u64 dmap_end)
> {
> @@ -693,10 +692,6 @@ int fuse_dax_break_layouts(struct inode *inode, u64 dmap_start,
> ret = __fuse_dax_break_layouts(inode, &retry, dmap_start,
> dmap_end);
> } while (ret == 0 && retry);
> - if (!dmap_end) {
> - dmap_start = 0;
> - dmap_end = LLONG_MAX;
> - }
>
> return ret;
> }
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 0b2f8567ca30..bc6c8936c529 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -1936,7 +1936,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> if (FUSE_IS_DAX(inode) && is_truncate) {
> filemap_invalidate_lock(mapping);
> fault_blocked = true;
> - err = fuse_dax_break_layouts(inode, 0, 0);
> + err = fuse_dax_break_layouts(inode, 0, -1);
> if (err) {
> filemap_invalidate_unlock(mapping);
> return err;
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 082ee374f694..cef7a8f75821 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -253,7 +253,7 @@ static int fuse_open(struct inode *inode, struct file *file)
>
> if (dax_truncate) {
> filemap_invalidate_lock(inode->i_mapping);
> - err = fuse_dax_break_layouts(inode, 0, 0);
> + err = fuse_dax_break_layouts(inode, 0, -1);
> if (err)
> goto out_inode_unlock;
> }
> @@ -2890,7 +2890,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
> inode_lock(inode);
> if (block_faults) {
> filemap_invalidate_lock(inode->i_mapping);
> - err = fuse_dax_break_layouts(inode, 0, 0);
> + err = fuse_dax_break_layouts(inode, 0, -1);
> if (err)
> goto out;
> }
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