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ircd::fs: Use GNU indirect function binding to efficiently support RWF_APPEND at runtime.

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This commit is contained in:
Jason Volk 2019-04-17 17:49:01 -07:00
parent 24b6f59a67
commit 6fbf882e16
1 changed files with 55 additions and 17 deletions
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72
ircd/fs.cc
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@ -63,8 +63,12 @@ ircd::fs::support_nowait
decltype(ircd::fs::support_append)
ircd::fs::support_append
{
info::kversion[0] >= 4 &&
info::kversion[1] >= 16
#if defined(HAVE_PWRITEV2) && defined(RWF_APPEND)
info::kversion[0] >= 4 &&
info::kversion[1] >= 16
#else
false
#endif
};
//
@ -774,6 +778,10 @@ ircd::fs::overwrite(const fd &fd,
return write(fd, bufs, opts);
}
//
// append
//
ircd::const_buffer
ircd::fs::append(const string_view &path,
const const_buffer &buf,
@ -819,29 +827,56 @@ ircd::fs::append(const string_view &path,
return append(fd, bufs, opts);
}
// When we have pwritev2() we can use RWF_APPEND indicated by
// the -1. Otherwise, we don't keep flags in userspace and we
// don't check the fd for whether it was opened with O_APPEND
// so the user may just have to eat the cost of an extra lseek().
#if defined(RWF_APPEND)
// Platform-specific append() implementations.
namespace ircd::fs
{
static size_t _append__rwf(const fd &, const const_buffers &, const write_opts &);
static size_t _append__no_rwf(const fd &, const const_buffers &, const write_opts &);
extern "C" decltype(_append__rwf) *ircd_fs_append__resolve();
}
size_t
ircd::fs::append(const fd &fd,
const const_buffers &bufs,
const write_opts &opts_)
__attribute__((ifunc("ircd_fs_append__resolve")));
/// This function allows the linker to dynamically resolve the append() definition
/// we'll be using during this execution. We check for fs::support_append which was
/// initialized based on the kernel version.
extern "C" decltype(ircd::fs::_append__rwf) *
ircd::fs::ircd_fs_append__resolve()
{
log::logf
{
log, support_append? log::DEBUG : log::DWARNING,
"This host '%s %s' %s the RWF_APPEND flag to pwritev2(2).",
info::kname,
info::kversion_str,
support_append? "SUPPORTS"_sv : "DOES NOT SUPPORT"_sv
};
return support_append? _append__rwf : _append__no_rwf;
}
/// When we have RWF_APPEND. We unconditionally set the offset to the
/// value -1.
size_t
ircd::fs::_append__rwf(const fd &fd,
const const_buffers &bufs,
const write_opts &opts_)
{
auto opts(opts_);
if(support_append)
opts.offset = -1;
else if(!opts.offset || opts.offset == -1)
opts.offset = syscall(::lseek, fd, 0, SEEK_END);
opts.offset = -1;
return write(fd, bufs, opts);
}
#else
/// When we don't have pwritev2() we have to eat the cost of an
/// extra lseek() to the end of the file.
size_t
ircd::fs::append(const fd &fd,
const const_buffers &bufs,
const write_opts &opts_)
ircd::fs::_append__no_rwf(const fd &fd,
const const_buffers &bufs,
const write_opts &opts_)
{
auto opts(opts_);
if(!opts.offset || opts.offset == -1)
@ -849,7 +884,10 @@ ircd::fs::append(const fd &fd,
return write(fd, bufs, opts);
}
#endif // RWF_APPEND
//
// write
//
ircd::const_buffer
ircd::fs::write(const string_view &path,