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SubjectShared mmap write-out (was: Re: patch cow-swapin)
On Sun, 27 Sep 1998, Andrea Arcangeli wrote:

> On Sun, 27 Sep 1998, Ion Badulescu wrote:
> >Here is a small program which will demonstrate the bug. Change the FILESIZE
> >value so it won't kill your machine, make sure you have enough disk space
> >(and no file called "foo"), then start it and watch the "bo" column of a
> >"vmstat 1".
> We are working on the "si" column. I' ve checked that the "si" column
> is always 0 with the new experimental smart swap cache code (2.1 only).

Right, I know. But the problem is similar, that is, the parent doesn't
know what the child is doing with pages in their shared mapping. This time
it's not even COW, it's a shared mapping altogether..

> Yes bo is full with 2.1 too, but I have not read your source and so I don'
> t know if this is the right behavior.

In short, here is what the program is doing:

1. create a large file
2. mmap it read/write shared
3. touch all pages in the mapping once, never touch them again
4. fork, then the child execs "/bin/echo", at which point the kernel
fsync's the large file (why? this is question no. 1) -> lots of disk
5. goto 4

So, question no. 2: why is the disk activity happening for *each* child,
although the pages are *never* touched again? They are part of the page
cache according to /proc/meminfo, so they should be marked non-dirty once
they're first flushed to the disk, and then nothing else should happen.


It is better to keep your mouth shut and be thought a fool,
than to open it and remove all doubt.

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