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Message-ID: <20200817172220.GB630630@redhat.com>
Date: Mon, 17 Aug 2020 13:22:20 -0400
From: Vivek Goyal <vgoyal@...hat.com>
To: Jan Kara <jack@...e.cz>
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 Mon, Aug 17, 2020 at 06:53:39PM +0200, Jan Kara wrote:
> 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..
Hi Jan,
Thanks for the review.
Will move kerneldoc comment.
>
> > -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.
Will do.
>
> > +
> > /*
> > * 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?
My bad. I forgot to change this. Previous version of patches had the
semantic that 'end == 0' signifies till the end of file. Yes, 'end'
should be LLONG_MAX now. Will fix it.
Thanks
Vivek
>
> Otherwise the patch looks good to me.
>
> Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
>
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