Date: Tue, 22 Sep 2009 13:54:28 +0400 From: Igor Sysoev <is@rambler-co.ru> To: Kostik Belousov <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org, d@delphij.net Subject: Re: fcntl(F_RDAHEAD) Message-ID: <20090922095428.GH1152@rambler-co.ru> In-Reply-To: <20090922085346.GR47688@deviant.kiev.zoral.com.ua> References: <20090917101526.GF57619@rambler-co.ru> <4AB2B7A1.5000601@delphij.net> <20090918074027.GI47688@deviant.kiev.zoral.com.ua> <20090921111245.GB23958@rambler-co.ru> <20090921112909.GQ47688@deviant.kiev.zoral.com.ua> <20090922072848.GA727@rambler-co.ru> <20090922085346.GR47688@deviant.kiev.zoral.com.ua>
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On Tue, Sep 22, 2009 at 11:53:46AM +0300, Kostik Belousov wrote: > On Tue, Sep 22, 2009 at 11:28:48AM +0400, Igor Sysoev wrote: > > On Mon, Sep 21, 2009 at 02:29:09PM +0300, Kostik Belousov wrote: > > > > > On Mon, Sep 21, 2009 at 03:12:45PM +0400, Igor Sysoev wrote: > > > > > > > What I dislike about the patch is the new kernel-private flag that is > > > > > eaten from the open(2) flags namespace. We do already have FHASLOCK, > > > > > so far the only such flag. > > > > > > > > We can change > > > > int f_seqcount; > > > > to > > > > u_int f_seqcount; > > > > > > > > and can use highest bit instead of O_READAHEAD: anyway f_seqcount is shifted > > > > to 16 bits left. > > > > > > Or do the same trick as was done for FHASLOCK and override some flag that > > > is not saved after open, see FMASK. > > > > > > Or split f_seqcount into two u_short fields, one for f_seqcount, second for > > > f_kflag, and use the later for FHASLOCK and FREADAHEAD. [We are trying to > > > not grow struct file unless absolutely neccessary]. > > > > I agree that struct file should not grow (at least in this case). > > However, I believe splitting f_seqcount into two fields will break > > kernel ABI. Or not ? I think f_seqcount should be splitted in 9-CURRENT > > and probably, in 8-STABLE, but in 7-STABLE we may use the open(2) flags > > namespace. > > The struct file indeed participates in the KBI, in particular, pointer > to it is supplied as an argument to VOP_OPEN() and d_fdopen(). On the > other hand, it is assumed that drivers and fses use it to override > f_ops and possibly f_data. f_seqcount status is internal VFS field that > probably should be not accessed or modified by driver or fs. > > Reason to try hard to keep layout of struct file intact even between major > branches is the userspace compatibility, with the code of lsof and fstat. > Might be, fstat will be improved to not require this. > > Probably, best temporal solution would be to override some flag used > only for open(2), postponing the task of separating bit- and name-spaces > for other day. Also, it makes merge to 8 and 7 easier. Well, I think O_CREAT or O_TRUNC are good candidate to be an alias for O_READAHEAD. -- Igor Sysoev http://sysoev.ru/en/
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