linux-kernel - Re: [RFC 2/8] shmem: convert to use folio_test

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Date:   Tue, 25 Apr 2023 13:00:25 +0200
From:   Pankaj Raghav <p.raghav@...sung.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Luis Chamberlain <mcgrof@...nel.org>, <hughd@...gle.com>,
        <akpm@...ux-foundation.org>, <brauner@...nel.org>,
        <djwong@...nel.org>, <da.gomez@...sung.com>,
        <a.manzanares@...sung.com>, <dave@...olabs.net>,
        <yosryahmed@...gle.com>, <keescook@...omium.org>, <hare@...e.de>,
        <kbusch@...nel.org>, <patches@...ts.linux.dev>,
        <linux-block@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <p.raghav@...sung.com>
Subject: Re: [RFC 2/8] shmem: convert to use folio_test_hwpoison()

On Fri, Apr 21, 2023 at 11:42:53PM +0100, Matthew Wilcox wrote:
> On Fri, Apr 21, 2023 at 02:43:54PM -0700, Luis Chamberlain wrote:
> > The PageHWPoison() call can be converted over to the respective folio call
> > folio_test_hwpoison(). This introduces no functional changes.
> 
> Um, no.  Nobody should use folio_test_hwpoison(), it's a nonsense.
> 
> Individual pages are hwpoisoned.  You're only testing the head page
> if you use folio_test_hwpoison().  There's folio_has_hwpoisoned() to
> test if _any_ page in the folio is poisoned.  But blindly converting
> PageHWPoison to folio_test_hwpoison() is wrong.

I see a pattern in shmem.c where first the head is tested and for large
folios, any of pages in the folio is tested for poison flag. Should we
factor it out as a helper in shmem.c and use it here?

static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
...
	if (folio_test_hwpoison(folio) ||
	    (folio_test_large(folio) &&
	     folio_test_has_hwpoisoned(folio))) {
	..
> 
> If anyone knows how to poison folio_test_hwpoison() to make it not
> work, I'd appreciate it.

IMO, I think it will be clear if folio_test_hwpoison checks if any of the
page in the folio is poisoned and we should have a explicit helper such
as folio_test_head_hwpoison if the callers want to only test if the head
page is poisoned (although I am not sure if that is useful).