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SubjectRe: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping
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On 7/2/18 11:09 PM, Michal Hocko wrote:
> On Mon 02-07-18 13:48:45, Andrew Morton wrote:
>> On Mon, 2 Jul 2018 16:05:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:
>>
>>> On Fri 29-06-18 20:15:47, Andrew Morton wrote:
>>> [...]
>>>> Would one of your earlier designs have addressed all usecases? I
>>>> expect the dumb unmap-a-little-bit-at-a-time approach would have?
>>> It has been already pointed out that this will not work.
>> I said "one of". There were others.
> Well, I was aware only about two potential solutions. Either do the
> heavy lifting under the shared lock and do the rest with the exlusive
> one and this, drop the lock per parts. Maybe I have missed others?

There is the other one which I presented on LSFMM summit. But, actually
it turns out that one looks very similar to the current under review one.

Yang

>
>>> You simply
>>> cannot drop the mmap_sem during unmap because another thread could
>>> change the address space under your feet. So you need some form of
>>> VM_DEAD and handle concurrent and conflicting address space operations.
>> Unclear that this is a problem. If a thread does an unmap of a range
>> of virtual address space, there's no guarantee that upon return some
>> other thread has not already mapped new stuff into that address range.
>> So what's changed?
> Well, consider the following scenario:
> Thread A = calling mmap(NULL, sizeA)
> Thread B = calling munmap(addr, sizeB)
>
> They do not use any external synchronization and rely on the atomic
> munmap. Thread B only munmaps range that it knows belongs to it (e.g.
> called mmap in the past). It should be clear that ThreadA should not
> get an address from the addr, sizeB range, right? In the most simple case
> it will not happen. But let's say that the addr, sizeB range has
> unmapped holes for what ever reasons. Now anytime munmap drops the
> exclusive lock after handling one VMA, Thread A might find its sizeA
> range and use it. ThreadB then might remove this new range as soon as it
> gets its exclusive lock again.
>
> Is such a code safe? No it is not and I would call it fragile at best
> but people tend to do weird things and atomic munmap behavior is
> something they can easily depend on.
>
> Another example would be an atomic address range probing by
> MAP_FIXED_NOREPLACE. It would simply break for similar reasons.
>
> I remember my attempt to make MAP_LOCKED consistent with mlock (if the
> population fails then return -ENOMEM) and that required to drop the
> shared mmap_sem and take it in exclusive mode (because we do not
> have upgrade_read) and Linus was strongly against [1][2] for very
> similar reasons. If you drop the lock you simply do not know what
> happened under your feet.
>
> [1] http://lkml.kernel.org/r/CA+55aFydkG-BgZzry5DrTzueVh9VvEcVJdLV8iOyUphQk=0vpw@mail.gmail.com
> [2] http://lkml.kernel.org/r/CA+55aFyajquhGhw59qNWKGK4dBV0TPmDD7-1XqPo7DZWvO_hPg@mail.gmail.com

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