Date: Sat, 21 Aug 2021 18:39:27 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Ka Ho Ng <khng@freebsd.org> Cc: freebsd-fs@freebsd.org Subject: Re: fspacectl(2): result of rmsr.r_offset for a success and non-partial operation Message-ID: <YSEeL6ivTeQBdEB%2B@kib.kiev.ua> In-Reply-To: <29087ddc-9b46-15fa-4041-c3a50dcf99db@FreeBSD.org> References: <87d4a87d-2ee3-74dd-3689-94dc0daf3983@FreeBSD.org> <29087ddc-9b46-15fa-4041-c3a50dcf99db@FreeBSD.org>
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On Fri, Aug 20, 2021 at 11:33:24PM +0800, Ka Ho Ng wrote: > On 2021/8/20 11:23 PM, Ka Ho Ng wrote: > > There were some recent discussion in https://reviews.freebsd.org/D31604 > > around the returned value of rmsr.r_offset. For a complete and > > successful operation, rmsr.r_len is set to 0. Regarding rmsr.r_offset, > > the bottom line is to have rmsr.r_offset being no greater than current > > file size in case rmsr.r_offset is greater than file size, while leaving > > rmsr.r_offset to be within EOF in case rqsr.r_offset is not beyond EOF. > > > > With the current approach, rmsr.r_offset is loosely defined as file > > system is free to set it to some value as long as it is neither smaller > > than rqsr.r_offset (in case rqsr.r_offset is within EOF), or beyond EOF > > (in case rqsr.r_offset is beyond EOF). Do you think it is a good idea to > > make it stricter in case the call succeeds and rmsr.r_len == 0 (i.e. a > > complete operation)? If that is the case, what if we set rmsr.r_offset > > to be rqsr.r_offset + rqsr.r_len? > > > > Ka Ho > > > > My another approach is to explicitly document that for a complete and > successful operation (i.e. rmsr.r_len == 0) callers need not to consider > the exact value of rmsr.r_offset, except if the operation range is not > complete outside of EOF, rmsr.r_offset is not going to be outside of EOF > either. I believe that for normal operation, rmsr.r_offset should be set to to rqsr.r_offset + rqsr.r_len, and for EOF case rqsr.r_offset + rqsr.r_len should be clipped at EOF. In both cases, rmsr.r_len should be set to zero. If rqsr.r_offset is beyond EOF, the statement above naturally implies that rmsr.r_offset is set to EOF, and rmsr.r_len is set to zero. This should interact well with a possibility that file is grown after the fspacectl(2) syscall is issued, and normal coding of the application using fspacectl().help
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