[lkml]   [2009]   [Aug]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    Subjectvma_merge issue

    I came across an issue where adjacent pages are not properly coalesced
    together when changing protections on them. This can be shown by doing
    the following:

    1) Map 3 pages with PROT_NONE and MAP_PRIVATE | MAP_ANONYMOUS
    2) Set the middle page's protection to PROT_READ | PROT_WRITE
    3) Set the middle page's protection back to PROT_NONE

    You are left with 3 entries in /proc/self/map where you should only
    have 1. If you only change the protection to PROT_READ in step 2, then
    it is properly merged together. I noticed in mprotect.c the following
    comment in the function mprotect_fixup; I'm not sure if it applies or
    * If we make a private mapping writable we increase our commit;
    * but (without finer accounting) cannot reduce our commit if we
    * make it unwritable again.
    * FIXME? We haven't defined a VM_NORESERVE flag, so mprotecting
    * a MAP_NORESERVE private mapping to writable will now reserve.

    I think this only applies to setting charged = nrpages; however,
    VM_ACCOUNT is also added to newflags. Could it be that the adjacent
    blocks don't have VM_ACCOUNT and so the call to vma_merge cannot merge
    because the flags for the adjacent vma are not the same?

    Can anyone shed some light on this? While it isn't an issue for 3
    pages, I'm mmaping 200K+ pages and changing the perms on random pages
    throughout and then back but I quickly run into the max_map_count when
    I don't actually need that many mappings.



     \ /
      Last update: 2009-08-10 22:49    [W:0.020 / U:4.176 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site