Date: Wed, 8 Dec 1999 14:38:48 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: Getting a new MAP_ flag into mmap() prior to 4.x freeze Message-ID: <199912082238.OAA43939@apollo.backplane.com> References: <Pine.BSF.4.21.9912081447110.4557-100000@fw.wintelcom.net>
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:I'd like to see this happen, go for it! :)
:
:Don't forget how getnewbuf refils the buffers though, it will need to
:somehow to communicate to the syncer to disregard MAP_NOSYNC during a
:shortage... ? :)
:
:-Alfred
No, I don't bother with that. If there is a filesystem buffer associated
with a dirty page the NOSYNC is ignored. The only time a filesystem
buffer can be associated with a NOSYNC page is if you write(). In that
case we allow the normal filesystem mechanisms to handle it.
The tie-in is really trivial -- there is essentially one procedure which
the object code calls to synchronize a range and it is comprised of two
parts: Collecting dirty pages and constructing filesystem buffers
for them, and flushing out filesystem buffers.
NOSYNC simply prevents the first part from occuring for normal
asynchronous flushes. The second part thus nevers sees the pages unless
some other command indirectly associates them with a buffer -- like write()
does for example.
For low-memory situations we let the pagedaemon handle things. The
pagedaemon ignores the NOSYNC flag. That is, NOSYNC space is treated
just the same as swap-backed memory is treated - pageed only when
necessary.
-Matt
Matthew Dillon
<dillon@backplane.com>
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