linux-kernel - Re: [PATCH v2 3/4] cachestat: implement cachestat syscall

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Date:   Mon, 12 Dec 2022 16:24:58 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, akpm@...ux-foundation.org,
        hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bfoster@...hat.com,
        kernel-team@...a.com
Subject: Re: [PATCH v2 3/4] cachestat: implement cachestat syscall

On Mon, Dec 12, 2022 at 08:23:31AM -0800, Nhat Pham wrote:
> > It would be easier to read if you inverted the condition here.
> 
> Oh I think I tried
> 
> if (!f.file)
>        return -EBADF;
> 
> here, but there are some mixing-code-with-decl warnings.
> If I recall correctly, the problem is with this line:
> 
> XA_STATE(xas, &mapping->i_pages, first_index);
> 
> which is expanded into a declaration:
> 
> #define XA_STATE(name, array, index) \
> struct xa_state name = __XA_STATE(array, index, 0, 0)
> 
> It requires a valid mapping though, which is
> obtained from f.file:
> 
> struct address_space *mapping = f.file->f_mapping;
> 
> so it cannot be moved above the if(!f.file) check either...

Perhaps you're trying to do too much in a single function?