cvs commit: src/sys/fs/nwfs nwfs_io.c src/sys/fs/smbfssmbfs_io.c src/sys/fs/specfs spec_vnops.c src/sys/kern uipc_syscalls.c vfs_bio.c src/sys/nfsclient nfs_bio.c src/sys/vm swap_page

Bruce Evans bde at
Thu May 6 08:30:35 PDT 2004

On Wed, 5 May 2004, Alan Cox wrote:

> alc         2004/05/05 22:03:24 PDT
>   FreeBSD src repository
>   Modified files:
>     sys/fs/nwfs          nwfs_io.c
>     sys/fs/smbfs         smbfs_io.c
>     sys/fs/specfs        spec_vnops.c
>     sys/kern             uipc_syscalls.c vfs_bio.c
>     sys/nfsclient        nfs_bio.c
>     sys/vm               swap_pager.c vm_fault.c vnode_pager.c
>   Log:
>   Make vm_page's PG_ZERO flag immutable between the time of the page's
>   allocation and deallocation.  This flag's principal use is shortly after
>   allocation.  For such cases, clearing the flag is pointless.  The only
>   unusual use of PG_ZERO is in vfs_bio_clrbuf().  However, allocbuf() never
>   requests a prezeroed page.  So, vfs_bio_clrbuf() never sees a prezeroed
                                                     ^^^^^ rarely(?)
>   page.
>   Reviewed by:    tegge@

The request for a prezeroed page is just a preference, so vfs_bio_clrbuf()
certainly gets prezeroed pages.  This happens whenever there are only
prezeroed pages to find.  I think vfs_bio_clrbuf() sees them too.  It
saw them at least once a few years ago when my debugging code for this
finally triggered.  My kernel at the time had colorizing optimizations
that probably made using a prezeroed page more likely.

The PG_ZERO optimizations in vfs_bio_clrbuf() were just worse than useless
because they rarely applied (and they break immutability here).


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