linux-kernel - Re: [PATCH v4] ceph: invalidate pages when doing direct/sync writes

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Date:   Fri, 8 Apr 2022 10:47:10 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Luís Henriques <lhenriques@...e.de>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] ceph: invalidate pages when doing direct/sync writes


On 4/7/22 11:15 PM, Luís Henriques wrote:
> When doing a direct/sync write, we need to invalidate the page cache in
> the range being written to.  If we don't do this, the cache will include
> invalid data as we just did a write that avoided the page cache.
>
> Signed-off-by: Luís Henriques <lhenriques@...e.de>
> ---
>   fs/ceph/file.c | 19 ++++++++++++++-----
>   1 file changed, 14 insertions(+), 5 deletions(-)
>
> Changes since v3:
> - Dropped initial call to invalidate_inode_pages2_range()
> - Added extra comment to document invalidation
>
> Changes since v2:
> - Invalidation needs to be done after a write
>
> Changes since v1:
> - Replaced truncate_inode_pages_range() by invalidate_inode_pages2_range
> - Call fscache_invalidate with FSCACHE_INVAL_DIO_WRITE if we're doing DIO
>
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 5072570c2203..97f764b2fbdd 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -1606,11 +1606,6 @@ ceph_sync_write(struct kiocb *iocb, struct iov_iter *from, loff_t pos,
>   		return ret;
>   
>   	ceph_fscache_invalidate(inode, false);
> -	ret = invalidate_inode_pages2_range(inode->i_mapping,
> -					    pos >> PAGE_SHIFT,
> -					    (pos + count - 1) >> PAGE_SHIFT);
> -	if (ret < 0)
> -		dout("invalidate_inode_pages2_range returned %d\n", ret);
>   
>   	while ((len = iov_iter_count(from)) > 0) {
>   		size_t left;
> @@ -1938,6 +1933,20 @@ ceph_sync_write(struct kiocb *iocb, struct iov_iter *from, loff_t pos,
>   			break;
>   		}
>   		ceph_clear_error_write(ci);
> +
> +		/*
> +		 * we need to invalidate the page cache here, otherwise the
> +		 * cache will include invalid data in direct/sync writes.
> +		 */
> +		ret = invalidate_inode_pages2_range(
> +				inode->i_mapping,
> +				pos >> PAGE_SHIFT,
> +				(pos + len - 1) >> PAGE_SHIFT);
> +		if (ret < 0) {
> +			dout("invalidate_inode_pages2_range returned %d\n",
> +			     ret);
> +			ret = 0;

For this, IMO it's not safe. If we just ignore it the pagecache will 
still have invalid data.

I think what the 'ceph_direct_read_write()' does is more correct, it 
will make sure all the dirty pages are writeback from the pagecaches by 
using 'invalidate_inode_pages2_range()' without blocking and later will 
do the invalidate blocked by using 'truncate_inode_pages_range()' if 
some pages are not unmaped in 'invalidate_inode_pages2_range()' when EBUSY.

This can always be sure that the pagecache has no invalid data after 
write finishes. I think why it use the truncate helper here is because 
it's safe and there shouldn't have any buffer write happen for DIO ?

But from my understanding the 'ceph_direct_read_write()' is still buggy. 
What if the page fault happen just after 'truncate_inode_pages_range()' 
? Will this happen ? Should we leave this to use the file lock to 
guarantee it in user space ?

Thought ?

-- Xiubo

> +		}
>   		pos += len;
>   		written += len;
>   		dout("sync_write written %d\n", written);
>