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Date: Thu, 20 Aug 2020 14:58:55 +0200
From: Jan Kara <jack@...e.cz>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-nvdimm@...ts.01.org, virtio-fs@...hat.com, miklos@...redi.hu,
stefanha@...hat.com, dgilbert@...hat.com, dan.j.williams@...el.com,
Jan Kara <jack@...e.cz>,
Vishal L Verma <vishal.l.verma@...el.com>,
"Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [PATCH v3 02/18] dax: Create a range version of
dax_layout_busy_page()
On Wed 19-08-20 18:19:40, 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
> Cc: Jan Kara <jack@...e.cz>
> Cc: Vishal L Verma <vishal.l.verma@...el.com>
> Cc: "Weiny, Ira" <ira.weiny@...el.com>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---
> fs/dax.c | 29 +++++++++++++++++++++++------
> include/linux/dax.h | 6 ++++++
> 2 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 95341af1a966..ddd705251d9f 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -559,7 +559,7 @@ static void *grab_mapping_entry(struct xa_state *xas,
> }
>
> /**
> - * dax_layout_busy_page - find first pinned page in @mapping
> + * dax_layout_busy_page_range - find first pinned page in @mapping
> * @mapping: address space to scan for a page with ref count > 1
Please document additional function arguments in the kernel-doc comment.
Otherwise the patch looks good so feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
after fixing this nit.
Honza
> *
> * DAX requires ZONE_DEVICE mapped pages. These pages are never
> @@ -572,13 +572,19 @@ static void *grab_mapping_entry(struct xa_state *xas,
> * 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.
> */
> -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;
> + XA_STATE(xas, &mapping->i_pages, start_idx);
>
> /*
> * In the 'limited' case get_user_pages() for dax is disabled.
> @@ -589,6 +595,11 @@ 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;
> + else
> + end_idx = end >> PAGE_SHIFT;
> /*
> * If we race get_user_pages_fast() here either we'll see the
> * elevated page count in the iteration and wait, or
> @@ -596,15 +607,15 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> * against is no longer mapped in the page tables and bail to the
> * get_user_pages() slow path. The slow path is protected by
> * pte_lock() and pmd_lock(). New references are not taken without
> - * holding those locks, and unmap_mapping_range() will not zero the
> + * holding those locks, and unmap_mapping_pages() will not zero the
> * pte or pmd without holding the respective lock, so we are
> * guaranteed to either see new references or prevent new
> * references from being established.
> */
> - unmap_mapping_range(mapping, 0, 0, 0);
> + unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1, 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 +636,12 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> xas_unlock_irq(&xas);
> return page;
> }
> +EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);
> +
> +struct page *dax_layout_busy_page(struct address_space *mapping)
> +{
> + return dax_layout_busy_page_range(mapping, 0, LLONG_MAX);
> +}
> EXPORT_SYMBOL_GPL(dax_layout_busy_page);
>
> static int __dax_invalidate_entry(struct address_space *mapping,
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 6904d4e0b2e0..9016929db4c6 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -141,6 +141,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
> struct dax_device *dax_dev, struct writeback_control *wbc);
>
> 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);
> dax_entry_t dax_lock_page(struct page *page);
> void dax_unlock_page(struct page *page, dax_entry_t cookie);
> #else
> @@ -171,6 +172,11 @@ static inline struct page *dax_layout_busy_page(struct address_space *mapping)
> return NULL;
> }
>
> +static inline struct page *dax_layout_busy_page_range(struct address_space *mapping, pgoff_t start, pgoff_t nr_pages)
> +{
> + return NULL;
> +}
> +
> static inline int dax_writeback_mapping_range(struct address_space *mapping,
> struct dax_device *dax_dev, struct writeback_control *wbc)
> {
> --
> 2.25.4
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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