Re: wrong madvise(MADV DONTNEED) semantic

["Michael Kerrisk" <mtk-lkml@gmx.net>]

> On Tue, 2005-06-28 at 11:16 -0700, Andy Isaacson wrote:
> 
> > Besides, if you read the documentation closely, it does not say what 
> > you
> > think it says.
> > 
> >        MADV_DONTNEED
> > 	      Do not expect access in the near future.  (For the time
> > 	      being, the application is finished with the given range,
> > 	      so the kernel can free resources associated with it.)
> > 	      Subsequent accesses of pages in this range will succeed,
> > 	      but will result either in reloading of the memory contents
> > 	      from the underlying mapped file (see mmap) or
> > 	      zero-fill-on-demand pages for mappings without an
> > 	      underlying file.
> > 
> > You seem to think that "reloading ... from the underlying mapped file"
> > means that changes are lost, but that's not implied.
> 
> This wording _does_ imply that changes are lost if the file is mapped
> writable and not mysnc'ed or if the memory mapping is anonymous.

A little late into this thread, but...

Indeed it does imply that, because that was what I understood
when (IIRC) I wrote that text in the man page.

> In the former, changes are dropped and the file is reread from the stale
> on-disk copy.  In the latter case, the data is dropped and the pages are
> zero-filled on access.

Yes.

Cheers,

Michael

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