linux-ext4 - Re: [PATCH v2 1/3] mm: add vm_normal_lru

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Date:   Thu, 31 Mar 2022 10:53:41 +0200
From:   Christoph Hellwig <hch@....de>
To:     Alex Sierra <alex.sierra@....com>
Cc:     jgg@...dia.com, david@...hat.com, Felix.Kuehling@....com,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v2 1/3] mm: add vm_normal_lru_pages for LRU handled
 pages only

> -	page = vm_normal_page(vma, addr, pte);
> +	page = vm_normal_lru_page(vma, addr, pte);

Why can't this deal with ZONE_DEVICE pages?  It certainly has
nothing do with a LRU I think.  In fact being able to have
stats that count say the number of device pages here would
probably be useful at some point.

In general I find the vm_normal_lru_page vs vm_normal_page
API highly confusing.  An explicit check for zone device pages
in the dozen or so spots that care has a much better documentation
value, especially if accompanied by comments where it isn't entirely
obvious.

>  		page = follow_page(vma, addr,
> -				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
> +				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE | FOLL_LRU);

Overly long line here.

> +/*
> + * NOTE: Technically this should goto check_pfn label. However, page->_mapcount
> + * is never incremented for device pages that are mmap through DAX mechanism
> + * using pmem driver mounted into ext4 filesystem. When these pages are unmap,
> + * zap_pte_range is called and vm_normal_page return a valid page with
> + * page_mapcount() = 0, before page_remove_rmap is called.
> + */

Please properly indent comments.

> + * zone, as long as the pte's are present and vm_normal_lru_page() succeeds. These
>   * pages also get pinned.

Another overly long line here.