Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to process_madvise

[Michal Hocko <mhocko@suse.com>]

On Fri 11-03-22 20:59:06, Charan Teja Kalla wrote:
> The process_madvise() system call is expected to skip holes in vma
> passed through 'struct iovec' vector list.

Where is this assumption coming from? From the man page I can see:
: The advice might be applied to only a part of iovec if one of its
: elements points to an invalid memory region in the remote
: process.  No further elements will be processed beyond that
: point.  

> But do_madvise, which
> process_madvise() calls for each vma, returns ENOMEM in case of unmapped
> holes, despite the VMA is processed.
> Thus process_madvise() should treat ENOMEM as expected and consider the
> VMA passed to as processed and continue processing other vma's in the
> vector list. Returning -ENOMEM to user, despite the VMA is processed,
> will be unable to figure out where to start the next madvise.

I am not sure I follow. With your previous patch and -ENOMEM from
do_madvise you get the the answer you are looking for, no?
With this applied you are loosing the information that some of the iters
are not mapped or has a hole. Which might be a useful information
especially when processing on remote tasks which are free to manipulate
their address spaces.

> Fixes: ecb8ac8b1f14("mm/madvise: introduce process_madvise() syscall: an external memory hinting API")
> Cc: <stable@vger.kernel.org> # 5.10+
> Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
> ---
> Changes in V2:
>   -- Fixed handling of ENOMEM by process_madvise().
>   -- Patch doesn't exist in V1.
> 
>  mm/madvise.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index e97e6a9..14fb76d 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1426,9 +1426,16 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  
>  	while (iov_iter_count(&iter)) {
>  		iovec = iov_iter_iovec(&iter);
> +		/*
> +		 * do_madvise returns ENOMEM if unmapped holes are present
> +		 * in the passed VMA. process_madvise() is expected to skip
> +		 * unmapped holes passed to it in the 'struct iovec' list
> +		 * and not fail because of them. Thus treat -ENOMEM return
> +		 * from do_madvise as valid and continue processing.
> +		 */
>  		ret = do_madvise(mm, (unsigned long)iovec.iov_base,
>  					iovec.iov_len, behavior);
> -		if (ret < 0)
> +		if (ret < 0 && ret != -ENOMEM)
>  			break;
>  		iov_iter_advance(&iter, iovec.iov_len);
>  	}
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs