linux-kernel - Re: [PATCH v1] fs/fuse: Fix missing FOLL

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Message-ID: <572dcce8-f70c-2d24-f844-a3e8abbd4bd8@fastmail.fm>
Date:   Tue, 29 Aug 2023 23:57:55 +0200
From:   Bernd Schubert <bernd.schubert@...tmail.fm>
To:     Lei Huang <lei.huang@...ux.intel.com>, linux-kernel@...r.kernel.org
Cc:     miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v1] fs/fuse: Fix missing FOLL_PIN for direct-io



On 8/29/23 20:36, Lei Huang wrote:
> Our user space filesystem relies on fuse to provide POSIX interface.
> In our test, a known string is written into a file and the content
> is read back later to verify correct data returned. We observed wrong
> data returned in read buffer in rare cases although correct data are
> stored in our filesystem.
> 
> Fuse kernel module calls iov_iter_get_pages2() to get the physical
> pages of the user-space read buffer passed in read(). The pages are
> not pinned to avoid page migration. When page migration occurs, the
> consequence are two-folds.
> 
> 1) Applications do not receive correct data in read buffer.
> 2) fuse kernel writes data into a wrong place.
> 
> Using iov_iter_extract_pages() to pin pages fixes the issue in our
> test.

Hmm, iov_iter_extract_pages does not exists for a long time and the code 
in fuse_get_user_pages didn't change much. So if you are right, there 
would be a long term data corruption for page migrations? And a back 
port to old kernels would not be obvious?

What confuses me further is that
commit 85dd2c8ff368 does not mention migration or corruption, although 
lists several other advantages for iov_iter_extract_pages. Other commits 
using iov_iter_extract_pages point to fork - i.e. would your data 
corruption be possibly related that?


Thanks,
Bernd


> 
> An auxiliary variable "struct page **pt_pages" is used in the patch
> to prepare the 2nd parameter for iov_iter_extract_pages() since
> iov_iter_get_pages2() uses a different type for the 2nd parameter.
> 
> Signed-off-by: Lei Huang <lei.huang@...ux.intel.com>
> ---
>   fs/fuse/file.c | 13 ++++++++-----
>   1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index bc41152..715de3b 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -670,7 +670,7 @@ static void fuse_release_user_pages(struct fuse_args_pages *ap,
>   	for (i = 0; i < ap->num_pages; i++) {
>   		if (should_dirty)
>   			set_page_dirty_lock(ap->pages[i]);
> -		put_page(ap->pages[i]);
> +		unpin_user_page(ap->pages[i]);
>   	}
>   }
>   
> @@ -1428,10 +1428,13 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>   	while (nbytes < *nbytesp && ap->num_pages < max_pages) {
>   		unsigned npages;
>   		size_t start;
> -		ret = iov_iter_get_pages2(ii, &ap->pages[ap->num_pages],
> -					*nbytesp - nbytes,
> -					max_pages - ap->num_pages,
> -					&start);
> +		struct page **pt_pages;
> +
> +		pt_pages = &ap->pages[ap->num_pages];
> +		ret = iov_iter_extract_pages(ii, &pt_pages,
> +					     *nbytesp - nbytes,
> +					     max_pages - ap->num_pages,
> +					     0, &start);
>   		if (ret < 0)
>   			break;
>