git: Kernel - Close VM/BIO races and document.o
dillon at crater.dragonflybsd.org
Thu Aug 27 22:49:15 PDT 2009
Author: Matthew Dillon <dillon at apollo.backplane.com>
Date: Thu Aug 27 20:34:50 2009 -0700
Kernel - Close VM/BIO races and document.o
* Remove vfs_setdirty(), it is no longer used.
Remove vfs_page_set_valid(), it is no longer used.
Remove vfs_bio_set_valid(), it is no longer used.
* When acquiring a buffer with getblk() whos size differs from the
buffer already cached, no longer destroy the VM pages backing
the buffer after completing the write. Instead just release
the buffer so a new, larger one can be constructed.
NFS buffers which straddle file EOF can remain cached after the
file has been extended via seek/write or ftruncate, and their
underlying VM pages may become dirty via mmap. If the buffer
is acquired later the underlying VM pages beyond the buffer's
original b_bcount size must be retained, not destroyed.
* No longer try to clear the pmap modified bit from misc vm_page_*()
functions. In cases where we desire the pmap modified bit to be
clear, it should *already* have been cleared in the run-up to the
I/O. Clearing it later may cause the buffer cache to lose track
of the fact that underlying VM pages may have been modified again.
NFS buffers use b_dirtyoff/b_dirtyend to determine what to actually
write. If the VM page is modified again the current write operation
will not cover all the dirty parts of the buffer and another write
will have to be issued. Clearing the pmap modified bit at later
stages did not properly track changes in b_dirtyoff/b_dirtyend and
resulted in dirty data being lost.
* Implement vfs_clean_one_page() to deal with nearly all buffer cache vs
backing VM page dirty->clean handling at the appropriate time.
In addition, this function now detects the case where a buffer has
B_NEEDCOMMIT set but the underlying VM page is dirty. This
function necessarily only clears the dirty bits associated
with the buffer because buffer sizes are not necessarily page aligned,
which is different from clearing ALL the dirty bits as the putpages
code is able to do. So the B_NEEDCOMMIT test is only against those
dirty bits associated with the buffer. If this is found to be the
case the B_NEEDCOMMIT flag is cleared.
This fixes a race where VM pages backing a dirty buffer which has gone
through the phase-1 commit are dirtied via a mmap, and NFS then goes
through with the phase-2 commit and throws the data away when it really
needed to go back and do another phase-1 commit.
* In vnode_generic_put_pages() no longer clear the VM page dirty bits
associated with bits of a file which extend past file EOF in the
page straddling the EOF. We used to do this with the idea that
we would only clear the dirty bits up to the file EOF later on
in the I/O completion code.
However, this was too fragile. If a page ended up with any dirty
bits left set it would remain endless dirty and be reflushed forever.
We now clear the dirty bits for the entire page after a putpages
operation completes without error, and don't bother doing it
prior to I/O initiation.
* Call nfs_meta_setsize() for both seek+write extensions (holes) and for
ftruncate extensions (holes).
nfs_meta_setsize() now deterministically adjusts the size of the buffer
that was straddling the PRIOR EOF point, fixing an issue where
write-extending a file to near the end of a nfs buffer boundary (32K),
then seek-write extending it further by creating a hole, then
mmap()ing the end of the first chunk and modifying data past the
original write-extend point... would lose the extra data because
the original buffer was still intact and was still sized for the
original EOF. This was difficult to reproduce because it only occurred
if all the dirty bits got cleared when the original buffer is flushed,
meaning the original write-extend point had to be within 511 bytes of
the end of a 32K boundary.
Summary of changes:
sys/kern/vfs_bio.c | 413 +++++++++++++++++++++----------------------
sys/vfs/devfs/devfs_vnops.c | 7 +-
sys/vfs/nfs/nfs.h | 3 +-
sys/vfs/nfs/nfs_bio.c | 97 +++++++++--
sys/vfs/nfs/nfs_vnops.c | 5 +-
sys/vfs/nwfs/nwfs_io.c | 9 +-
sys/vfs/smbfs/smbfs_io.c | 9 +-
sys/vm/vm_page.c | 19 ++-
sys/vm/vm_page2.h | 4 +
sys/vm/vnode_pager.c | 34 ++--
10 files changed, 345 insertions(+), 255 deletions(-)
DragonFly BSD source repository
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