Re: [PATCH] 2.6.14 patch for supporting madvise(MADV_FREE)

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Nov 01, 2005 at 05:15:01PM -0800, Badari Pulavarty wrote:
> Here is the patch to support madvise(MADV_FREE) - which frees 
> up the given range of pages and truncates the underlying backing 
> store. This basically provides "punch hole into file" functionality.
> Currently it supports ONLY shmfs/tmpfs - where we have short term 
> need. Other filesystems return -ENOSYS.

MADV_FREE as a name isn't right if we return -ENOSYS for anonymoys
memory.

MADV_FREE in other OS works _only_ on anonymous memory and returns
-EINVAL if used on filebacked vmas. Infact we probably should rename our
MADV_DONTNEED to MADV_FREE.

http://docs.sun.com/app/docs/doc/816-5168/6mbb3hrde?a=view

	"This value cannot be used on mappings that have underlying file objects."

Our MADV_DONTNEED exactly matches the MADV_FREE semantics, and it seems
the MADV_DONTNEED of other OS isn't destructive like ours. Except our
MADV_DONTNEED also works on filebacked mappings but it's destructive
only on anonymous memory.


I thought Andrew suggested MADV_REMOVE for the new feature.

This feature didn't exist in other OS yet AFIK, so a new MADV_name for
it makes sense. I'm not completely against extending MADV_FREE but then we
shouldn't return -ENOSYS on anonymous memory and we should do the same
thing MADV_DONTNEED does on anonymous memory. Probably a new name is
safer to avoid confusion (think an application running MADV_FREE and
expecting -EINVAL when used on filebacked mappings).

Thanks!