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Date: Mon, 17 Aug 2020 18:53:39 +0200
From: Jan Kara <jack@...e.cz>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
virtio-fs@...hat.com, miklos@...redi.hu, stefanha@...hat.com,
dgilbert@...hat.com, Dan Williams <dan.j.williams@...el.com>,
linux-nvdimm@...ts.01.org
Subject: Re: [PATCH v2 02/20] dax: Create a range version of
dax_layout_busy_page()
On Fri 07-08-20 15:55:08, Vivek Goyal wrote:
> virtiofs device has a range of memory which is mapped into file inodes
> using dax. This memory is mapped in qemu on host and maps different
> sections of real file on host. Size of this memory is limited
> (determined by administrator) and depending on filesystem size, we will
> soon reach a situation where all the memory is in use and we need to
> reclaim some.
>
> As part of reclaim process, we will need to make sure that there are
> no active references to pages (taken by get_user_pages()) on the memory
> range we are trying to reclaim. I am planning to use
> dax_layout_busy_page() for this. But in current form this is per inode
> and scans through all the pages of the inode.
>
> We want to reclaim only a portion of memory (say 2MB page). So we want
> to make sure that only that 2MB range of pages do not have any
> references (and don't want to unmap all the pages of inode).
>
> Hence, create a range version of this function named
> dax_layout_busy_page_range() which can be used to pass a range which
> needs to be unmapped.
>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: linux-nvdimm@...ts.01.org
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
The API looks OK. Some comments WRT the implementation below.
> diff --git a/fs/dax.c b/fs/dax.c
> index 11b16729b86f..0d51b0fbb489 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -558,27 +558,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
> return xa_mk_internal(VM_FAULT_FALLBACK);
> }
>
> -/**
> - * dax_layout_busy_page - find first pinned page in @mapping
> - * @mapping: address space to scan for a page with ref count > 1
> - *
> - * DAX requires ZONE_DEVICE mapped pages. These pages are never
> - * 'onlined' to the page allocator so they are considered idle when
> - * page->count == 1. A filesystem uses this interface to determine if
> - * any page in the mapping is busy, i.e. for DMA, or other
> - * get_user_pages() usages.
> - *
> - * It is expected that the filesystem is holding locks to block the
> - * establishment of new mappings in this address_space. I.e. it expects
> - * to be able to run unmap_mapping_range() and subsequently not race
> - * mapping_mapped() becoming true.
> +/*
> + * Partial pages are included. If end is LLONG_MAX, pages in the range from
> + * start to end of the file are inluded.
> */
I think the big kerneldoc comment should stay with
dax_layout_busy_page_range() since dax_layout_busy_page() will be just a
trivial wrapper around it..
> -struct page *dax_layout_busy_page(struct address_space *mapping)
> +struct page *dax_layout_busy_page_range(struct address_space *mapping,
> + loff_t start, loff_t end)
> {
> - XA_STATE(xas, &mapping->i_pages, 0);
> void *entry;
> unsigned int scanned = 0;
> struct page *page = NULL;
> + pgoff_t start_idx = start >> PAGE_SHIFT;
> + pgoff_t end_idx = end >> PAGE_SHIFT;
> + XA_STATE(xas, &mapping->i_pages, start_idx);
> + loff_t len, lstart = round_down(start, PAGE_SIZE);
>
> /*
> * In the 'limited' case get_user_pages() for dax is disabled.
> @@ -589,6 +582,22 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> return NULL;
>
> + /* If end == LLONG_MAX, all pages from start to till end of file */
> + if (end == LLONG_MAX) {
> + end_idx = ULONG_MAX;
> + len = 0;
> + } else {
> + /* length is being calculated from lstart and not start.
> + * This is due to behavior of unmap_mapping_range(). If
> + * start is say 4094 and end is on 4096 then we want to
> + * unamp two pages, idx 0 and 1. But unmap_mapping_range()
> + * will unmap only page at idx 0. If we calculate len
> + * from the rounded down start, this problem should not
> + * happen.
> + */
> + len = end - lstart + 1;
> + }
Maybe it would be more understandable to use
unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1);
below and avoid all this rounding and special-casing.
> +
> /*
> * If we race get_user_pages_fast() here either we'll see the
> * elevated page count in the iteration and wait, or
> @@ -601,10 +610,10 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> * guaranteed to either see new references or prevent new
> * references from being established.
> */
> - unmap_mapping_range(mapping, 0, 0, 0);
> + unmap_mapping_range(mapping, start, len, 0);
>
> xas_lock_irq(&xas);
> - xas_for_each(&xas, entry, ULONG_MAX) {
> + xas_for_each(&xas, entry, end_idx) {
> if (WARN_ON_ONCE(!xa_is_value(entry)))
> continue;
> if (unlikely(dax_is_locked(entry)))
> @@ -625,6 +634,27 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> xas_unlock_irq(&xas);
> return page;
> }
> +EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);
> +
> +/**
> + * dax_layout_busy_page - find first pinned page in @mapping
> + * @mapping: address space to scan for a page with ref count > 1
> + *
> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> + * 'onlined' to the page allocator so they are considered idle when
> + * page->count == 1. A filesystem uses this interface to determine if
> + * any page in the mapping is busy, i.e. for DMA, or other
> + * get_user_pages() usages.
> + *
> + * It is expected that the filesystem is holding locks to block the
> + * establishment of new mappings in this address_space. I.e. it expects
> + * to be able to run unmap_mapping_range() and subsequently not race
> + * mapping_mapped() becoming true.
> + */
> +struct page *dax_layout_busy_page(struct address_space *mapping)
> +{
> + return dax_layout_busy_page_range(mapping, 0, 0);
Should the 'end' rather be LLONG_MAX?
Otherwise the patch looks good to me.
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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