linux-ext4 - Re: [RFC 04/10] fs/dax: Don't track page mapping/index

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Date: Fri, 12 Apr 2024 17:22:08 +0200
From: Jan Kara <jack@...e.cz>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, david@...morbit.com, dan.j.williams@...el.com,
	jhubbard@...dia.com, rcampbell@...dia.com, willy@...radead.org,
	jgg@...dia.com, linux-fsdevel@...r.kernel.org, jack@...e.cz,
	djwong@...nel.org, hch@....de, david@...hat.com,
	ruansy.fnst@...itsu.com, nvdimm@...ts.linux.dev,
	linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	jglisse@...hat.com
Subject: Re: [RFC 04/10] fs/dax: Don't track page mapping/index

On Thu 11-04-24 10:57:25, Alistair Popple wrote:
> The page->mapping and page->index fields are normally used by the
> pagecache and rmap for looking up virtual mappings of pages. FS DAX
> implements it's own kind of page cache and rmap look ups so these
> fields are unnecessary. They are currently only used to detect
> error/warning conditions which should never occur.
> 
> A future change will change the way shared mappings are detected by
> doing normal page reference counting instead, so remove the
> unnecessary checks.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
...
> -/*
> - * When it is called in dax_insert_entry(), the shared flag will indicate that
> - * whether this entry is shared by multiple files.  If so, set the page->mapping
> - * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
> - */
> -static void dax_associate_entry(void *entry, struct address_space *mapping,
> -		struct vm_area_struct *vma, unsigned long address, bool shared)
> -{
> -	unsigned long size = dax_entry_size(entry), pfn, index;
> -	int i = 0;
> -
> -	if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> -		return;
> -
> -	index = linear_page_index(vma, address & ~(size - 1));
> -	for_each_mapped_pfn(entry, pfn) {
> -		struct page *page = pfn_to_page(pfn);
> -
> -		if (shared) {
> -			dax_page_share_get(page);
> -		} else {
> -			WARN_ON_ONCE(page->mapping);
> -			page->mapping = mapping;
> -			page->index = index + i++;
> -		}
> -	}
> -}

Hum, but what about existing uses of folio->mapping and folio->index in
fs/dax.c? AFAICT this patch breaks them. What am I missing? How can this
ever work?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR