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From: Jan Kara <jack@suse.cz>
To: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	cluster-devel@redhat.com, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH v3 1/7] iov_iter: Introduce fault_in_iov_iter helper
Date: Mon, 26 Jul 2021 18:33:26 +0200	[thread overview]
Message-ID: <20210726163326.GK20621@quack2.suse.cz> (raw)
In-Reply-To: <20210723205840.299280-2-agruenba@redhat.com>

On Fri 23-07-21 22:58:34, Andreas Gruenbacher wrote:
> Introduce a new fault_in_iov_iter helper for manually faulting in an iterator.
> Other than fault_in_pages_writeable(), this function is non-destructive.
> 
> We'll use fault_in_iov_iter in gfs2 once we've determined that the iterator
> passed to .read_iter or .write_iter isn't in memory.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
...
> +unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
> +				  bool write)
> +{
> +	struct mm_struct *mm = current->mm;
> +	struct vm_area_struct *vma = NULL;
> +	unsigned long end, nstart, nend;
> +	int locked = 0;
> +	int gup_flags;
> +
> +	/*
> +	 * FIXME: Make sure this function doesn't succeed for pages that cannot
> +	 * be accessed; otherwise we could end up in a loop trying to fault in
> +	 * and then access the pages.  (It's okay if a page gets evicted and we
> +	 * need more than one retry.)
> +	 */
> +
> +	/*
> +	 * FIXME: Are these the right FOLL_* flags?
> +	 */

How about the FIXMEs here? I guess we should answer these questions before
merging and remove the comments. Regarding the first FIXME I tend to agree
that if we cannot fault in the first page, we should return the error
rather than returning 0 as you do now. OTOH the caller can check for 0 and
understand there's something wrong going on as well. But the error would be
probably a bit clearer.

> +
> +	gup_flags = FOLL_TOUCH | FOLL_POPULATE;

I don't think FOLL_POPULATE makes sense here. It makes sense only with
FOLL_MLOCK and determines whether mlock(2) should fault in missing pages or
not.

								Honza

> +	if (write)
> +		gup_flags |= FOLL_WRITE;
> +
> +	end = PAGE_ALIGN(start + len);
> +	for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
> +		unsigned long nr_pages;
> +		long ret;
> +
> +		if (!locked) {
> +			locked = 1;
> +			mmap_read_lock(mm);
> +			vma = find_vma(mm, nstart);
> +		} else if (nstart >= vma->vm_end)
> +			vma = vma->vm_next;
> +		if (!vma || vma->vm_start >= end)
> +			break;
> +		nend = min(end, vma->vm_end);
> +		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
> +			continue;
> +		if (nstart < vma->vm_start)
> +			nstart = vma->vm_start;
> +		nr_pages = (nend - nstart) / PAGE_SIZE;
> +		ret = __get_user_pages_locked(mm, nstart, nr_pages,
> +					      NULL, NULL, &locked, gup_flags);
> +		if (ret <= 0)
> +			break;
> +		nend = nstart + ret * PAGE_SIZE;
> +	}
> +	if (locked)
> +		mmap_read_unlock(mm);
> +	if (nstart > start)
> +		return min(nstart - start, len);
> +	return 0;
> +}
> +
>  /**
>   * get_dump_page() - pin user page in memory while writing it to core dump
>   * @addr: user address
> -- 
> 2.26.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  parent reply	other threads:[~2021-07-26 16:40 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-23 20:58 [PATCH v3 0/7] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-07-23 20:58 ` [PATCH v3 1/7] iov_iter: Introduce fault_in_iov_iter helper Andreas Gruenbacher
2021-07-23 23:40   ` Linus Torvalds
2021-07-24  7:51     ` Andreas Grünbacher
2021-07-24  1:52   ` Al Viro
2021-07-24  8:05     ` Andreas Grünbacher
2021-07-26 16:33   ` Jan Kara [this message]
2021-07-26 17:15     ` Linus Torvalds
2021-07-23 20:58 ` [PATCH v3 2/7] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-07-23 20:58 ` [PATCH v3 3/7] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-07-23 20:58 ` [PATCH v3 4/7] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-07-23 20:58 ` [PATCH v3 5/7] iomap: Support restarting direct I/O requests after user copy failures Andreas Gruenbacher
2021-07-26 17:19   ` Jan Kara
     [not found]     ` <CAHpGcMLtQ1=WOT1mTUS4=iWBwHLQ-EBzY=+XuSGJfu4gVPYTLw@mail.gmail.com>
2021-07-26 18:08       ` Jan Kara
2021-07-23 20:58 ` [PATCH v3 6/7] iov_iter: Introduce noio flag to disable page faults Andreas Gruenbacher
2021-07-23 20:58 ` [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-07-26 17:02   ` Jan Kara
     [not found]     ` <CAHpGcMLOZhZ7tGrY7rcYWUwx12sY884T=eC-Ckna63PBmF=zwA@mail.gmail.com>
2021-07-26 18:00       ` Jan Kara

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