Re: [RFC PATCH 0/6] mm: use augmented rbtrees for finding unmapped areas

On Wed, Oct 31, 2012 at 3:33 AM, Michel Lespinasse [off-list ref] wrote:

My own feel for this series is that I'm fairly confident in the
robustness of my vm_unmapped_area() implementation; however I would
like to confirm that people are happy with this new interface. Also
the code that figures out what constraints to pass to
vm_unmapped_area() is a bit odd; I have tried to make the constraints
match the behavior of the current code but it's not clear to me if
that behavior makes sense in the first place.

I wanted to expand a bit on that by listing some of these behaviors I
have made sure to preserve without really understanding why they are
as they are:

- arch_get_unmapped_area() doesn't make use of mm->mmap_base, this
value is used only when doing downwards allocations. However, many
architectures including x86_64 carefully initialize this (in
arch_pick_mmap_layout() ) to different values based on the up/down
allocation direction. It seems that the legacy (upwards allocation)
mmap_base value is irrelevant as I don't see any place using it ???

- For downwards allocations, it is not clear if the lowest valid
address should be 0 or PAGE_SIZE. Existing brute-force search code
will treat address 0 as valid on entering the loop, but invalid when
reaching the end of the loop.

- When user passes a suggested address without the MAP_FIXED flag, the
address range we validate the address against varies depending on the
upwards/downwards allocation direction. This doesn't make much sense
since there is no address space search taking place in this case.

- The stragegy of allocating upwards if the downwards allocation
failed is a bit strange. I'm not sure what we really want; maybe we
only need to extend the valid address range for the initial search ?
(IIRC Rik's initial patch series got rid of this redundant search, but
didn't explain why this was considered safe).

That's all I noticed, but this is really most of the remaining code
left in arch_get_unmapped_area[_topdown]... and I didn't even go into
architectures other than x86, where I could find some additional
questionable stuff (but I don't even want to go there before we at
least agree on the general principle of this patch series).

I hope with a proper understanding of the allocation strategies /
constraints it might be possible to unify the remaining
arch_get_unmapped_area[_topdown] code between architectures, but I'm
keeping this for a later step as I'm obviously not informed enough to
tackle that just yet...

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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