Date: Mon, 18 Jan 1999 17:28:35 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: "John S. Dyson" <root@dyson.iquest.net> Cc: dg@root.com, jkh@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Re: Found problem w/ Paging performance over NFS Message-ID: <199901190128.RAA84794@apollo.backplane.com> References: <199901190120.UAA27272@dyson.iquest.net>
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:>
:> in kern/vfs_bio.c, allocbuf()
:>
:> } else if (m->flags & PG_BUSY) {
:> s = splvm();
:> if (m->flags & PG_BUSY) {
:> vm_page_flag_set(m, PG_WANTED);
:> tsleep(m, PVM, "pgtblk", 0);
:> }
:> splx(s);
:> goto doretry;
:> } else {
:> ...
:> if (tinc > (newbsize - toff))
:> tinc = newbsize - toff;
:> if (bp->b_flags & B_CACHE)
:> vfs_buf_set_valid(bp, off, toff, tinc, m);
:> vm_page_flag_clear(m, PG_ZERO);
:> vm_page_wire(m);
:> }
:>
:> Shouldn't those conditionals be 'm->busy || (m->flags & PG_BUSY)' instead
:> of just testing against PG_BUSY? A pageout operation will set m->busy
:> without setting PG_BUSY.
:>
:The existing code is okay. Pageouts don't need to set PG_BUSY. Also, *generally*
:during buffer cache read write operations, the PG_BUSY flag doesn't need to be set
:during the *entire* operation. Think filesystems with block sizes < PAGE_SIZE
:as an example.
:
:PG_BUSY is quite severe, and not always needed.
:
:John
Ok. I just figured out the interaction -- in fact, we *can't* test m->busy
in this section of code because of the following sequence:
vm_fault -> vnode_pager_getpages -> ffs_getpages -> ffs_read ->
cluster_read -> getblk -> allocbuf ( lockup )
ffs_getpages converts the PG_BUSY into a vm_page_t->busy before getting into
the cluster stuff, and thus allocbuf() deep down cannot sleep on
vm_page_t->busy.
Ick. I'm going to add a comment explaining that case.
-Matt
Matthew Dillon
<dillon@backplane.com>
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