On Fri, 5 Aug 2005, Prasanna Meda wrote:
> Hugh Dickins wrote:
>
> > 2. Correct initial value of prev when starting part way into a vma: as
> > in sys_mprotect and do_mlock, it needs to be set to vma in this case
> > (vma_merge handles only that minimum of cases shown in its comments).
>
> Acknowledge corrections 1 and 3 readily. Treated vma_merge
> as block box that can handle all cases. Motivation for pointless
> case 3 is to skip holes and did not notice that has been covered.
> Thanks for corrections.
And thanks for the confirmations.
> Correction 2 is tricky. Sometimes it merges similar to case 3,
> misses a needed split, where after the fix we can get case 4
> merge. If that is what you are saying, we are in agreement.
> Otherwise, can you explain the real problem?
I probably am saying what you are saying there,
but it's hard for me to understand it that way.
Missing out the "start > vma->vm_start" adjustment of prev introduces
additional (but redundant: non-canonical) cases not considered at all
by vma_merge, now entered with a "prev" which is remote and surely
irrelevant to merging. "misses a needed split", yes, I saw that;
indeed my test ended up taking the "cases 3, 8" path, when, given
the right prev, it should have been handled as a "case 4".
mmap(0x80000000, 0x3000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, fd, 0);
madvise(0x80000000, 0x2000, MADV_RANDOM);
madvise(0x80001000, 0x1000, MADV_NORMAL);
ended up (if I'm remembering it aright) as one single VM_RAND_READ vma,
when it should have been a one page VM_RAND_READ vma followed by a two
page normal vma. Merged into one because of the incorrect prev, and
VM_RAND_READ rather than normal because of the misplaced success label.
Hugh
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