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

From: Vlastimil Babka
Date: Mon Aug 24 2015 - 09:30:40 EST

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

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?

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)?
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