Re: [PATCH] mm: fix false-positive OVERCOMMIT_GUESS failures

[Vlastimil Babka]

On 4/12/19 10:06 PM, Roman Gushchin wrote:
> On Fri, Apr 12, 2019 at 03:14:18PM -0400, Johannes Weiner wrote:
>> With the default overcommit==guess we occasionally run into mmap
>> rejections despite plenty of memory that would get dropped under
>> pressure but just isn't accounted reclaimable. One example of this is
>> dying cgroups pinned by some page cache. A previous case was auxiliary
>> path name memory associated with dentries; we have since annotated
>> those allocations to avoid overcommit failures (see d79f7aa496fc ("mm:
>> treat indirectly reclaimable memory as free in overcommit logic")).
>>
>> But trying to classify all allocated memory reliably as reclaimable
>> and unreclaimable is a bit of a fool's errand. There could be a myriad
>> of dependencies that constantly change with kernel versions.

Just wondering, did you find at least one another reclaimable case like
those path names?

>> It becomes even more questionable of an effort when considering how
>> this estimate of available memory is used: it's not compared to the
>> system-wide allocated virtual memory in any way. It's not even
>> compared to the allocating process's address space. It's compared to
>> the single allocation request at hand!
>>
>> So we have an elaborate left-hand side of the equation that tries to
>> assess the exact breathing room the system has available down to a
>> page - and then compare it to an isolated allocation request with no
>> additional context. We could fail an allocation of N bytes, but for
>> two allocations of N/2 bytes we'd do this elaborate dance twice in a
>> row and then still let N bytes of virtual memory through. This doesn't
>> make a whole lot of sense.
>>
>> Let's take a step back and look at the actual goal of the
>> heuristic. From the documentation:
>>
>>    Heuristic overcommit handling. Obvious overcommits of address
>>    space are refused. Used for a typical system. It ensures a
>>    seriously wild allocation fails while allowing overcommit to
>>    reduce swap usage.  root is allowed to allocate slightly more
>>    memory in this mode. This is the default.
>>
>> If all we want to do is catch clearly bogus allocation requests
>> irrespective of the general virtual memory situation, the physical
>> memory counter-part doesn't need to be that complicated, either.
>>
>> When in GUESS mode, catch wild allocations by comparing their request
>> size to total amount of ram and swap in the system.
>>
>> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> My 2c here: any kinds of percpu counters and percpu data is accounted
> as unreclaimable and can alter the calculation significantly.
> 
> This is a special problem on hosts, which were idle for some time.
> Without any memory pressure, kernel caches do occupy most of the memory,
> so than a following attempt to start a workload fails.

So then we remove the kmalloc-reclaimable caches again as not worth the
trouble anymore (they might be useful for anti-fragmentation purposes,
but that's much harder to quantify), or what?

> With a big pleasure:
> Acked-by: Roman Gushchin <guro@xxxxxx>
> 
> Thanks!
>