linux-kernel - Re: [PATCH 2/2] mm: madvise: return exact bytes advised with process

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Date:   Thu, 24 Mar 2022 21:15:57 +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>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 2/2] mm: madvise: return exact bytes advised with
 process_madvise under error

Thanks Michal for the inputs.

On 3/24/2022 6:44 PM, Michal Hocko wrote:
> On Wed 23-03-22 20:54:10, Charan Teja Kalla wrote:
>> From: Charan Teja Reddy <quic_charante@...cinc.com>
>>
>> The commit 5bd009c7c9a9 ("mm: madvise: return correct bytes advised with
>> process_madvise") fixes the issue to return number of bytes that are
>> successfully advised before hitting error with iovec elements
>> processing. But, when the user passed unmapped ranges in iovec, the
>> syscall ignores these holes and continues processing and returns ENOMEM
>> in the end, which is same as madvise semantic. This is a problem for
>> vector processing where user may want to know how many bytes were
>> exactly processed in a iovec element to make better decissions in the
>> user space. As in ENOMEM case, we processed all bytes in a iovec element
>> but still returned error which will confuse the user whether it is
>> failed or succeeded to advise.
> Do you have any specific example where the initial semantic is really
> problematic or is this mostly a theoretical problem you have found when
> reading the code?
> 
> 
>> As an example, consider below ranges were passed by the user in struct
>> iovec: iovec1(ranges: vma1), iovec2(ranges: vma2 -- vma3 -- hole) and
>> iovec3(ranges: vma4). In the current implementation, it fully advise
>> iovec1 and iovec2 but just returns number of processed bytes as iovec1
>> range. Then user may repeat the processing of iovec2, which is already
>> processed, which then returns with ENOMEM. Then user may want to skip
>> iovec2 and starts processing from iovec3. Here because of wrong return
>> processed bytes, iovec2 is processed twice.
> I think you should be much more specific why this is actually a problem.
> This would surely be less optimal but is this a correctness issue?
> 

Yes, this is a problem found when reading the code, but IMO we can
easily expect an invalid vma/hole in the passed range because we are
operating on other process VMA. More than solving the problem of being
less optimal, this can be looked in the direction of helping the user to
take better policy decisions with this syscall. And, not better policy
decisions from user is just being sub optimal(i.e. issuing the syscall
again on the processed range) with this syscall.

Having said that, at present I don't have any reports/unit test showing
the existing semantic is really a problematic.

> [...]
>> +	vma = find_vma_prev(mm, start, &prev);
>> +	if (vma && start > vma->vm_start)
>> +		prev = vma;
>> +
>> +	blk_start_plug(&plug);
>> +	for (;;) {
>> +		/*
>> +		 * It it hits a unmapped address range in the [start, end),
>> +		 * stop processing and return ENOMEM.
>> +		 */
>> +		if (!vma || start < vma->vm_start) {
>> +			error = -ENOMEM;
>> +			goto out;
>> +		}
>> +
>> +		tmp = vma->vm_end;
>> +		if (end < tmp)
>> +			tmp = end;
>> +
>> +		error = madvise_vma_behavior(vma, &prev, start, tmp, behavior);
>> +		if (error)
>> +			goto out;
>> +		tmp_bytes_advised += tmp - start;
>> +		start = tmp;
>> +		if (prev && start < prev->vm_end)
>> +			start = prev->vm_end;
>> +		if (start >= end)
>> +			goto out;
>> +		if (prev)
>> +			vma = prev->vm_next;
>> +		else
>> +			vma = find_vma(mm, start);
>> +	}
>> +out:
>> +	/*
>> +	 * partial_bytes_advised may contain non-zero bytes indicating
>> +	 * the number of bytes advised before failure. Holds zero incase
>> +	 * of success.
>> +	 */
>> +	*partial_bytes_advised = error ? tmp_bytes_advised : 0;
> Although this looks like a fix I am not sure it is future proof.
> madvise_vma_behavior doesn't report which part of the range has been
> really processed. I do not think that currently supported madvise modes
> for process_madvise support an early break out with return to the
> userspace (madvise_cold_or_pageout_pte_range bails on fatal signals for
> example) but this can change in the future and then you are back to
> "imprecise" return value problem. Yes, this is a theoretical problem

Agree here with the "imprecise" return value problem with processing a
VMA range. Yes when it is decided to return proper processed value from
madvise_vma_behavior(), this code too may need the maintenance.

> but so it sounds the problem you are trying to fix IMHO. I think it
> would be better to live with imprecise return values reporting rather
> than aiming for perfection which would be fragile and add a future
> maintenance burden.
>
Hmm. Should atleast this imprecise return values be documented in man
page or in madvise.c file?

> On the other hand if there are _real_ workloads which suffer from the
> existing semantic then sure the above seems to be an appropriate fix
> AFAICS.


> -- Michal Hocko SUSE Labs