On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
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.
Oh, right... Well that looks like a convincing argument for having a
sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
existing pte's would slow it down, and be unreliable (was the area
completely populated because MLOCK_ONFAULT was not used or because the
process aulted it already? Was it not populated because MLOCK_ONFAULT
was used, or because mmap(MAP_LOCKED) failed to populate it all?).
The only sane alternative is to populate always for mremap() of
VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information as
a limitation of mlock2(MLOCK_ONFAULT). Which might or might not be
enough for Eric's usecase, but it's somewhat ugly.
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.
Maybe, but please do it as a separate series.
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