Date: Fri, 15 Apr 2011 09:22:18 -0700 From: mdf@FreeBSD.org To: Kostik Belousov <kostikbel@gmail.com> Cc: FreeBSD Arch <freebsd-arch@freebsd.org> Subject: Re: posix_fallocate(2) Message-ID: <BANLkTi=rX-JAzF8-ueNg9yFnZKyKqGgxfg@mail.gmail.com> In-Reply-To: <20110415093057.GJ48734@deviant.kiev.zoral.com.ua> References: <BANLkTimYzJ11w9X1OHShEn2wi6gjHx=YjA@mail.gmail.com> <20110414213610.GB92382@tops> <BANLkTi=OWUnB_ue3RT4bzGNvivZwW_ofkA@mail.gmail.com> <20110415093057.GJ48734@deviant.kiev.zoral.com.ua>
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2011/4/15 Kostik Belousov <kostikbel@gmail.com>: > On Thu, Apr 14, 2011 at 03:41:30PM -0700, mdf@freebsd.org wrote: >> On Thu, Apr 14, 2011 at 2:36 PM, Gleb Kurtsou <gleb.kurtsou@gmail.com> w= rote: >> > On (14/04/2011 12:35), mdf@FreeBSD.org wrote: >> >> For work we need a functionality in our filesystem that is pretty muc= h >> >> like posix_fallocate(2), so we're using the name and I've added a >> >> default VOP_ALLOCATE definition that does the right, but dumb, thing. >> >> >> >> The most recent mention of this function in FreeBSD was another threa= d >> >> lamenting it's failure to exist: >> >> http://lists.freebsd.org/pipermail/freebsd-ports/2010-February/059268= .html >> >> >> >> The attached files are the core of the kernel implementation of the >> >> syscall and a default VOP for any filesystem not supporting >> >> VOP_ALLOCATE, which allows the syscall to work as expected but in a >> >> non-performant manner. =A0I didn't see this syscall in NetBSD or >> >> OpenBSD, so I plan to add it to the end of our syscall table. >> >> >> >> What I wanted to check with -arch about was: >> >> >> >> 1) is there still a desire for this syscall? >> > It looks not to play well architecturally with modern COW file systems >> > like ZFS and HUMMER. So potentially it can be implemented only for UFS= . >> >> The syscall, or the dumb implementation? =A0I don't see why the syscall >> itself would be a problem; presumably ZFS can figure out whether an >> fallocate() block is worth COWing or not... >> >> >> 2) is this naive implementation useful enough to serve as a default >> >> for all filesystems until someone with more knowledge fills them in? >> > Maillist ate the patch. Only man page attached. >> >> Whoops! >> >> http://people.freebsd.org/~mdf/bsd-fallocate.diff > > New syscall symbols for 9.0 should go in under FBSD_1.2 version, not FBSD= _1.0. Okay, fixed. > You have inconsistent spacing in the kern_posix_fallocate(). Oops; copy/paste error; fixed. > I do not quite understand the locking for vnode you did. > You marked the vop as taking and returning unlocked vnode. But, you > do call VOP_GETATTR in the vop std implementation before locking the vnod= e. > Did you tested with DEBUG_VFS_LOCKS config ? I have mostly tested on the version of FreeBSD we run at work which has some small KPI modifications. I will test and fix up on CURRENT once I figure out prove(1). As for locking: (1) For $WORK FreeBSD's locking of a "File" is problematic since we have both an inode lock and a data lock, and lots of times we don't really need the inode locked exclusively, just the data, which we handle inside the VOP. (2) I don't want to make 1TB allocated in a single operation, under a single lock, so the implementation is responsible for unlocking and taking a breather as needed. (3) I based the VOP_GETATTR on vn_stat which calls VOP_GETATTR without any lock. Except, hmm, it looks like vn_statfile(9) takes the lock. I was trying to avoid a lock/unlock cycle when the file didn't need to be extended, but I can put it back in. > Usual (and proper) practice is to have such vop require locked vnode, in > case of VOP_ALLOCATE, exclusive lock is appropriate. The Giant dance and > vn_start_write() + vn_lock() go into kern_posix_fallocate() then. > Also, you should call bwillwrite() before taking any vfs locks. > > Is locking/unlocking the vnode in loop is done to allow other callers > to perform i/o on the vnode in between ? In particular, to truncate it ? > I think this is not needed, and previous suggestion would take care of it= . See above; it is not acceptable in my mind to lock the vnode for the entire length of the operation, so the locking is managed by the VOP. > Why do you need stdallocate_extend() ? VOP_WRITE does the right thing > with extending the vnode. I was trying to simplify the implementation to a easy read/write loop since it isn't supposed to be performant but just get the right data. I could instead VOP_GETATTR on each loop to check file size and write zeros past the current file size, but that was more logic than a single VOP_SETATTR followed by read/write. > You might find vn_rdwr easier to use then the bare vops. In particular, > it would not omit the mac calls for read/write. I checked for write already in kern_posix_fallocate(). A single check should be sufficient. For other threads, please note I don't know anything about UFS implementations and I can't provide a ufs_allocte() that does rapid allocation of logically zero blocks. My intent is to provide the framework, a default implementation that meets the spec'd behaviour, and a set of testcases suitable to run for any filesystem that wants to verify their implementation. Thanks, matthew
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