linux-kernel - Re: [PATCH v3] 9p: fix EBADF errors in cached mode

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Date:   Mon, 20 Jun 2022 14:47:24 +0200
From:   Christian Schoenebeck <linux_oss@...debyte.com>
To:     Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        David Howells <dhowells@...hat.com>, stable@...r.kernel.org
Cc:     v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] 9p: fix EBADF errors in cached mode

On Donnerstag, 16. Juni 2022 23:10:25 CEST Dominique Martinet wrote:
> cached operations sometimes need to do invalid operations (e.g. read
> on a write only file)
> Historic fscache had added a "writeback fid", a special handle opened
> RW as root, for this. The conversion to new fscache missed that bit.
> 
> This commit reinstates a slightly lesser variant of the original code
> that uses the writeback fid for partial pages backfills if the regular
> user fid had been open as WRONLY, and thus would lack read permissions.
> 
> Link:
> https://lkml.kernel.org/r/20220614033802.1606738-1-asmadeus@codewreck.org
> Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do reads
> and caching") Cc: stable@...r.kernel.org
> Cc: David Howells <dhowells@...hat.com>
> Reported-By: Christian Schoenebeck <linux_oss@...debyte.com>
> Reviewed-by: Christian Schoenebeck <linux_oss@...debyte.com>
> Tested-by: Christian Schoenebeck <linux_oss@...debyte.com>
> Signed-off-by: Dominique Martinet <asmadeus@...ewreck.org>
> ---
> v3: use the least permissive version of the patch that only uses
> writeback fid when really required
> 
> If no problem shows up by then I'll post this patch around Wed 23 (next
> week) with the other stable fixes.
> 
>  fs/9p/vfs_addr.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
> index a8f512b44a85..d0833fa69faf 100644
> --- a/fs/9p/vfs_addr.c
> +++ b/fs/9p/vfs_addr.c
> @@ -58,8 +58,21 @@ static void v9fs_issue_read(struct netfs_io_subrequest
> *subreq) */
>  static int v9fs_init_request(struct netfs_io_request *rreq, struct file
> *file) {
> +	struct inode *inode = file_inode(file);
> +	struct v9fs_inode *v9inode = V9FS_I(inode);
>  	struct p9_fid *fid = file->private_data;
> 
> +	BUG_ON(!fid);
> +
> +	/* we might need to read from a fid that was opened write-only
> +	 * for read-modify-write of page cache, use the writeback fid
> +	 * for that */
> +	if (rreq->origin == NETFS_READ_FOR_WRITE &&
> +			(fid->mode & O_ACCMODE) == O_WRONLY) {
> +		fid = v9inode->writeback_fid;
> +		BUG_ON(!fid);
> +	}
> +
>  	refcount_inc(&fid->count);
>  	rreq->netfs_priv = fid;
>  	return 0;

Some more tests this weekend; all looks fine. It appears that this also fixed
the performance degradation that I reported early in this thread. Again,
benchmarks compiling a bunch of sources:

Case  Linux kernel version         msize   cache  duration (average)

A)    EBADF fix only [1]           512000  loose  31m 14s
B)    EBADF fix only [1]           512000  mmap   44m 1s
C)    EBADF fix + clunk fixes [2]  512000  loose  29m 32s
D)    EBADF fix + clunk fixes [2]  512000  mmap   44m 0s
E)    5.10.84                      512000  loose  35m 5s
F)    5.10.84                      512000  mmap   65m 5s

[1] 5.19.0-rc2 + EBADF fix v3 patch (alone):
https://lore.kernel.org/lkml/20220616211025.1790171-1-asmadeus@codewreck.org/

[2] 5.19.0-rc2 + EBADF fix v3 patch + clunk fix patches, a.k.a. 9p-next:
https://github.com/martinetd/linux/commit/b0017602fdf6bd3f344dd49eaee8b6ffeed6dbac

Conclusion: all thumbs in my possession pointing upwards. :)

Thanks Dominique!

Best regards,
Christian Schoenebeck