Date: Wed, 22 Jan 2003 13:44:41 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Garrett Wollman <wollman@lcs.mit.edu> Cc: peter@wemm.org, arch@FreeBSD.ORG Subject: Re: getsysfd() patch #1 (Re: Virtual memory question) Message-ID: <200301222144.h0MLifiC009829@apollo.backplane.com> References: <200301220304.h0M34TMB099694@apollo.backplane.com> <200301222052.h0MKqP4N041427@khavrinen.lcs.mit.edu>
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You guys are talking apples and oranges here. shm_open() is a libc
call, not a system call. getmemfd() or equivalent is a system call
and performs a function that no other system call can duplicate. Until
we fix our VFS layering system to provide a call vector for file
descriptors, the descriptors returned by the two APIs are not going to
be compatible. e.g. you can call ftruncate() on an shm_open()
descriptor (because it is essentially just a file descriptor), but you
cannot call ftruncate() on a getmemfd() descriptor.
The primary problem with using shm_open() in its current implementation
is the fact that the anonymous memory associated with the descriptor
is file-backed when what we really want is swap-backed anonymous memory.
file-backed memory has serious repercussions that swap backed memory does
not. The two most serious repercussions are (A) it will eat space in
the filesystem and (B) if you do not pre-zero the space, any pageout
activity will (due to its randomness) create an extremely inefficient,
highly fragmented file. Additionally, the most important madvise()
function, MADV_FREE, does not work on file-backed memory.
-Matt
Matthew Dillon
<dillon@backplane.com>
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:In article <20030122041546.393AF2A8A5@canning.wemm.org> you write:
:
:>I'd rather do that than add another arg to the memfd syscall. Otherwise
:>we're heading into shm_open() look and feel territory. Hmm. I wonder if
:>the POSIX folks would object to us calling the syscall shm_openfd() ?
:
:Can you please explain just what precisely your problem is with
:shm_open()? If we're going to reinvent the wheel, please, let's skip
:the square and hexagonal stages.
:
:- -GAWollman
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