Date: Tue, 29 Sep 1998 11:14:03 -0700 (PDT) From: Matthew Dillon <dillon@backplane.com> To: cvs-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG Subject: A second mmap file corruption issue Message-ID: <199809291814.LAA20823@apollo.backplane.com>
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I've verified that the *other* mmap file corruption issue that
Terry brought up is real.
Here's what happened to me:
-stable system news1.best.com
All of diablo's mmap's are read-only shared.
Diablo is operating fine, no problems.
I kill Diablo and restart it.
Suddenly, an mmap'd portion of diablo's history file is corrupted,
containing a portion of some other file.
Diablo shares pages and mmap's up the wazoo. What I believe happened was
that all the page references in the mmap were hiding a wiring count
failure in the bp code. When I exited diablo, I believe that a bp wound
up with b_pages[] pointing to pages that were no longer wired and then
the pages got reused.
This is conjecture. I would like someone to look at the following code,
though:
(1) around line 659 of kern/vfs_bio.c, in brelse(). I think the page
needs to be wired !!!!!!! Won't the wire count get corrupted
when the bp is released and the page isn't wired ?
if (m == bogus_page) {
m = vm_page_lookup(obj, poff + j);
...
bp->b_pages[j] = m;
}
(2) around line 1969 of kern/vfs_bio.c, in biodone. I think this
page needs to be wired as well.
if (m == bogus_page) {
bogusflag = 1;
m = vm_page_lookup(obj, OFF_TO_IDX(foff));
...
bp->b_pages[i] = m;
pmap_qenter(trunc_page(bp->b_data), bp->b_pages, bp->b_npages);
}
(3) Could someone also check vfs_busy_pages() and vfs_unbusy_pages() ?
busy_pages removes pages from the bp without unwiring them,
presumably because unbusy_pages adds them back without wiring
them. That seems rather fragile to me but I *think* the code is
doing the right thing as long as all vfs_busy_pages() calls are
matched by vfs_unbusy_pages() calls.
-Matt
Matthew Dillon Engineering, HiWay Technologies, Inc. & BEST Internet
Communications & God knows what else.
<dillon@backplane.com> (Please include original email in any response)
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