Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

From: Konstantin Khlebnikov
Date: Mon Aug 24 2015 - 09:50:50 EST

On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>>> I am in the middle of implementing lock on fault this way, but I cannot
>>> see how we will hanlde mremap of a lock on fault region. Say we have
>>> the following:
>>> addr = mmap(len, MAP_ANONYMOUS, ...);
>>> mlock(addr, len, MLOCK_ONFAULT);
>>> ...
>>> mremap(addr, len, 2 * len, ...)
>>> There is no way for mremap to know that the area being remapped was lock
>>> on fault so it will be locked and prefaulted by remap. How can we avoid
>>> this without tracking per vma if it was locked with lock or lock on
>>> fault?
>> remap can count filled ptes and prefault only completely populated areas.
> Does (and should) mremap really prefault non-present pages? Shouldn't it
> just prepare the page tables and that's it?

As I see mremap prefaults pages when it extends mlocked area.

Also quote from manpage
: If the memory segment specified by old_address and old_size is locked
: (using mlock(2) or similar), then this lock is maintained when the segment is
: resized and/or relocated. As a consequence, the amount of memory locked
: by the process may change.

>> There might be a problem after failed populate: remap will handle them
>> as lock on fault. In this case we can fill ptes with swap-like non-present
>> entries to remember that fact and count them as should-be-locked pages.
> I don't think we should strive to have mremap try to fix the inherent
> unreliability of mmap (MAP_POPULATE)?

I don't think so. MAP_POPULATE works only when mmap happens.
Flag MREMAP_POPULATE might be a good idea. Just for symmetry.
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