linux-kernel - Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v2

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Message-ID: <20200901174216.GT14765@casper.infradead.org>
Date:   Tue, 1 Sep 2020 18:42:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: remove the last set_fs() in common code, and remove it for x86
 and powerpc v2

On Tue, Sep 01, 2020 at 06:25:12PM +0100, Al Viro wrote:
> On Tue, Sep 01, 2020 at 07:13:00PM +0200, Christophe Leroy wrote:
> 
> >     10.92%  dd       [kernel.kallsyms]  [k] iov_iter_zero
> 
> Interesting...  Could you get an instruction-level profile inside iov_iter_zero(),
> along with the disassembly of that sucker?

Also, does [1] make any difference?  Probably not since it's translating
O flags into IOCB flags instead of RWF flags into IOCB flags.  I wonder
if there's a useful trick we can play here ... something like:

static inline int iocb_flags(struct file *file)
{
        int res = 0;
	if (likely(!file->f_flags & O_APPEND | O_DIRECT | O_DSYNC | __O_SYNC)) && !IS_SYNC(file->f_mapping->host))
		return res;
        if (file->f_flags & O_APPEND)
                res |= IOCB_APPEND;
        if (file->f_flags & O_DIRECT)
                res |= IOCB_DIRECT;
        if ((file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host))
                res |= IOCB_DSYNC;
        if (file->f_flags & __O_SYNC)
                res |= IOCB_SYNC;
        return res;
}

Can we do something like force O_DSYNC to be set if the inode IS_SYNC()
at the time of open?  Or is setting the sync bit on the inode required
to affect currently-open files?

[1] https://lore.kernel.org/linux-fsdevel/95de7ce4-9254-39f1-304f-4455f66bf0f4@kernel.dk/