Re: [PATCH] mm/mmap: fix the adjusted length error

[Michel Lespinasse]

I worry that the proposed change turns the search from an O(log N)
worst case into a O(N) one.

To see why the current search is O(log N), it is easiest to start by
imagining a simplified search algorithm that wouldn't include the low
and high address limits. In that algorithm, backtracking through the
vma tree is never necessary - the tree walk can always know, prior to
going left or right, if a suitable gap will be found in the
corresponding subtree.

The code we have today does have to respect the low and high address
limits, so it does need to implement backtracking - but this
backtracking only occurs to back out of subtrees that include the low
address limit (the search went 'left' into a subtree that has a large
enough gap, but the gap turns out to be below the limit so it can't be
used and the search needs to go 'right' instead). Because of this, the
amount of backtracking that can occur is very limited, and the search
is still O(log N) in the worst case.

With your proposed change, backtracking could occur not only around
the low address limit, but also at any node within the search tree,
when it turns out that a gap that seemed large enough actually isn't
due to alignment constraints. So, the code should still work, but it
could backtrack more in the worst case, turning the worst case search
into an O(N) thing.

I am not sure what to do about this. First I would want to understand
more about your test case; is this something that you stumbled upon
without expecting it or was it an artificially constructed case to
show the limitations of the current search algorithm ? Also, if your
process does something unusual and expects to be able to map (close
to) the entirety of its address space, would it be reasonable for it
to manually manage the address space and pass explicit addresses to
mmap / shmat ?

On Thu, May 16, 2019 at 11:02 PM jianhong chen <chenjianhong2@xxxxxxxxxx> wrote:
> In linux version 4.4, a 32-bit process may fail to allocate 64M hugepage
> memory by function shmat even though there is a 64M memory gap in
> the process.
>
> It is the adjusted length that causes the problem, introduced from
> commit db4fbfb9523c935 ("mm: vm_unmapped_area() lookup function").
> Accounting for the worst case alignment overhead, function unmapped_area
> and unmapped_area_topdown adjust the search length before searching
> for available vma gap. This is an estimated length, sum of the desired
> length and the longest alignment offset, which can cause misjudgement
> if the system has very few virtual memory left. For example, if the
> longest memory gap available is 64M, we canât get it from the system
> by allocating 64M hugepage memory via shmat function. The reason is
> that it requires a longger length, the sum of the desired length(64M)
> and the longest alignment offset.
>
> To fix this error ,we can calculate the alignment offset of
> gap_start or gap_end to get a desired gap_start or gap_end value,
> before searching for the available gap. In this way, we don't
> need to adjust the search length.
>
> Problem reproduces procedure:
> 1. allocate a lot of virtual memory segments via shmat and malloc
> 2. release one of the biggest memory segment via shmdt
> 3. attach the biggest memory segment via shmat
>
> e.g.
> process maps:
> 00008000-00009000 r-xp 00000000 00:12 3385    /tmp/memory_mmap
> 00011000-00012000 rw-p 00001000 00:12 3385    /tmp/memory_mmap
> 27536000-f756a000 rw-p 00000000 00:00 0
> f756a000-f7691000 r-xp 00000000 01:00 560     /lib/libc-2.11.1.so
> f7691000-f7699000 ---p 00127000 01:00 560     /lib/libc-2.11.1.so
> f7699000-f769b000 r--p 00127000 01:00 560     /lib/libc-2.11.1.so
> f769b000-f769c000 rw-p 00129000 01:00 560     /lib/libc-2.11.1.so
> f769c000-f769f000 rw-p 00000000 00:00 0
> f769f000-f76c0000 r-xp 00000000 01:00 583     /lib/libgcc_s.so.1
> f76c0000-f76c7000 ---p 00021000 01:00 583     /lib/libgcc_s.so.1
> f76c7000-f76c8000 rw-p 00020000 01:00 583     /lib/libgcc_s.so.1
> f76c8000-f76e5000 r-xp 00000000 01:00 543     /lib/ld-2.11.1.so
> f76e9000-f76ea000 rw-p 00000000 00:00 0
> f76ea000-f76ec000 rw-p 00000000 00:00 0
> f76ec000-f76ed000 r--p 0001c000 01:00 543     /lib/ld-2.11.1.so
> f76ed000-f76ee000 rw-p 0001d000 01:00 543     /lib/ld-2.11.1.so
> f7800000-f7a00000 rw-s 00000000 00:0e 0       /SYSV000000ea (deleted)
> fba00000-fca00000 rw-s 00000000 00:0e 65538   /SYSV000000ec (deleted)
> fca00000-fce00000 rw-s 00000000 00:0e 98307   /SYSV000000ed (deleted)
> fce00000-fd800000 rw-s 00000000 00:0e 131076  /SYSV000000ee (deleted)
> ff913000-ff934000 rw-p 00000000 00:00 0       [stack]
> ffff0000-ffff1000 r-xp 00000000 00:00 0       [vectors]
>
> from 0xf7a00000 to fba00000, it has 64M memory gap, but we can't get
> it from kernel.