linux-kernel - Re: [PATCH v4 07/11] iomap: fix iomap_dio

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Date: Sat, 27 Apr 2024 04:26:44 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>, djwong@...nel.org,
	brauner@...nel.org, david@...morbit.com, chandan.babu@...cle.com,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	hare@...e.de, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-xfs@...r.kernel.org, mcgrof@...nel.org, gost.dev@...sung.com,
	p.raghav@...sung.com
Subject: Re: [PATCH v4 07/11] iomap: fix iomap_dio_zero() for fs bs > system
 page size

On Thu, Apr 25, 2024 at 11:22:35PM -0700, Christoph Hellwig wrote:
> On Thu, Apr 25, 2024 at 01:37:42PM +0200, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@...sung.com>
> > 
> > iomap_dio_zero() will pad a fs block with zeroes if the direct IO size
> > < fs block size. iomap_dio_zero() has an implicit assumption that fs block
> > size < page_size. This is true for most filesystems at the moment.
> > 
> > If the block size > page size, this will send the contents of the page
> > next to zero page(as len > PAGE_SIZE) to the underlying block device,
> > causing FS corruption.
> > 
> > iomap is a generic infrastructure and it should not make any assumptions
> > about the fs block size and the page size of the system.
> 
> So what happened to the plan to making huge_zero_page a folio and have

There's a series of commits in linux-mm with the titles:

      sparc: use is_huge_zero_pmd()
      mm: add is_huge_zero_folio()
      mm: add pmd_folio()
      mm: convert migrate_vma_collect_pmd to use a folio
      mm: convert huge_zero_page to huge_zero_folio
      mm: convert do_huge_pmd_anonymous_page to huge_zero_folio
      dax: use huge_zero_folio
      mm: rename mm_put_huge_zero_page to mm_put_huge_zero_folio

> it available for non-hugetlb setups?  Not only would this be cleaner
> and more efficient, but it would actually work for the case where you'd
> have to zero more than 1MB on a 4k PAGE_SIZE system, which doesn't
> seem impossible with 2MB folios.

It is available for non-hugetlb setups.  It is however allocated on
demand, so it might not be available.