Re: [RFC] Strange do_munmap in mmap_region

[Wang YanQing <udknight@gmail.com>]

On Thu, Mar 19, 2015 at 11:33:41AM +0300, Konstantin Khlebnikov wrote:
> On Sat, Feb 28, 2015 at 9:46 AM, Wang YanQing <udknight@gmail.com> wrote:
> > Hi Mel Gorman and all.
> >
> > I have read do_mmap_pgoff and mmap_region more than one hour,
> > but still can't catch sense about below code in mmap_region:
> >
> > "
> >         /* Clear old maps */
> >         error = -ENOMEM;
> > munmap_back:
> >         if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) {
> >                 if (do_munmap(mm, addr, len))
> >                         return -ENOMEM;
> >                 goto munmap_back;
> >         }
> > "
> >
> > How can we just do_munmap overlapping vma without check its vm_flags
> > and new vma's vm_flags? I must miss some important things, but I can't
> > figure out.
> >
> > You give below comment about the code in "understand the linux memory manager":)
> >
> > "
> > If a VMA was found and it is part of the new mmapping, this removes the
> >  old mmapping because the new one will cover both
> > "
> >
> > But if new mmapping has different vm_flags or others' property, how
> > can we just say the new one will cover both?
> >
> > I appreicate any clue and explanation about this headache question.
> >
> > Thanks.
> >
> 
> Mmap() creates new mapping in given range
> (new vma might be merged to one or both of sides if possible)
> so everything what was here before is unmapped in process. Not?

Thanks for reply.

Assme process has vma in region 4096-8192, one page size, mapped to
a file's first 4096 bytes, then a new map want to create vma in range
0-8192 to map 4096-1288 in file, please tell me what's your meaning:
"so everything what was here before is unmapped in process"?

Why we can just delete old vma for first 4096 size in file which reside
in range 4096-8192 without notify user process? And create the new vma
to occupy range 0-8192, do you think "everything" is really the same?

Process lost old map for file's first 4096 bytes, and we use a new
map for 4096-1288 in file to lie it, and say "the same".

Indeed, I have another question, I guess the answer could save me the
same as this question.

I have read get_unmapped_area, it seems it will return a unused enough
region for new vma, and we hold mm->mmap_sem before vm_mmap_pgoff,
why unused enough region return by get_unmapped_area has overlapping vma
in mmap_region cause the first question?

I have tested it, running system always call do_munmap in mmap_region, so
I must miss something important, it is strange.

Thanks again.
> 
> >
> >
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