linux-kernel - Re: [PATCH 1/2] Revert "mm: madvise: skip unmapped vma holes passed to process

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Date:   Thu, 24 Mar 2022 19:33:56 +0530
From:   Charan Teja Kalla <quic_charante@...cinc.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     <akpm@...ux-foundation.org>, <minchan@...nel.org>,
        <surenb@...gle.com>, <vbabka@...e.cz>, <rientjes@...gle.com>,
        <nadav.amit@...il.com>, <edgararriaga@...gle.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] Revert "mm: madvise: skip unmapped vma holes passed
 to process_madvise"

Thanks Michal.

On 3/24/2022 6:18 PM, Michal Hocko wrote:
> On Wed 23-03-22 20:54:09, Charan Teja Kalla wrote:
>> This reverts commit 08095d6310a7 ("mm: madvise: skip unmapped vma holes
>> passed to process_madvise") as process_madvise() fails to return exact
>> processed bytes at other cases too. As an example: if the
>> process_madvise() hits mlocked pages after processing some initial bytes
>> passed in [start, end), it just returns EINVAL though some bytes are
>> processed. Thus making an exception only for ENOMEM is partially fixing
>> the problem of returning the proper advised bytes.
>>
>> Thus revert this patch and return proper bytes advised, if there any,
>> for all the error types in the following patch.
> 
> I do agree with the revert. I am not sure the above really is a proper
> justification though. 08095d6310a7 was changing one (arguably) dubious
> semantic by another one without a proper justification and wider
> consensus which I would expect from a patch which changes an existing
> semantic. Not to mention it being marked for stable tree.

Thanks for pointing this out. Since 08095d6310a7 is marked for stable
tree, doing the same for this change.

Cc: <stable@...r.kernel.org> # 5.10+

> 
> But let's not nit pick on that now. Let's send this revert ASAP and use
> some more time to discuss the semantic and whether any change is really
> required.
> 
>> Signed-off-by: Charan Teja Kalla <quic_charante@...cinc.com>
> 
> Acked-by: Michal Hocko <mhocko@...e.com>
> 
Thanks for the quick ack.
>> ---
>>  mm/madvise.c | 9 +--------
>>  1 file changed, 1 insertion(+), 8 deletions(-)
>>
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index 39b712f..0d8fd17 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -1433,16 +1433,9 @@ 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 && ret != -ENOMEM)
>> +		if (ret < 0)
>>  			break;
>>  		iov_iter_advance(&iter, iovec.iov_len);
>>  	}
>> -- 
>> 2.7.4
>