Re: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze)

[Mike Rapoport <rppt@kernel.org>]

On Mon, May 03, 2021 at 10:28:36AM +0200, David Hildenbrand wrote:
> On 02.05.21 08:34, Mike Rapoport wrote:
> > On Thu, Apr 29, 2021 at 02:25:19PM +0200, David Hildenbrand wrote:
> > > Let's properly synchronize with drivers that set PageOffline(). Unfreeze
> > > every now and then, so drivers that want to set PageOffline() can make
> > > progress.
> > > 
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > ---
> > >   fs/proc/kcore.c | 15 +++++++++++++++
> > >   1 file changed, 15 insertions(+)
> > > 
> > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> > > index 92ff1e4436cb..3d7531f47389 100644
> > > --- a/fs/proc/kcore.c
> > > +++ b/fs/proc/kcore.c
> > > @@ -311,6 +311,7 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
> > >   static ssize_t
> > >   read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
> > >   {
> > > +	size_t page_offline_frozen = 0;
> > >   	char *buf = file->private_data;
> > >   	size_t phdrs_offset, notes_offset, data_offset;
> > >   	size_t phdrs_len, notes_len;
> > > @@ -509,6 +510,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
> > >   			pfn = __pa(start) >> PAGE_SHIFT;
> > >   			page = pfn_to_online_page(pfn);
> > 
> > Can't this race with page offlining for the first time we get here?
> 
> 
> To clarify, we have three types of offline pages in the kernel ...
> 
> a) Pages part of an offline memory section; the memap is stale and not
> trustworthy. pfn_to_online_page() checks that. We *can* protect against
> memory offlining using get_online_mems()/put_online_mems(), but usually
> avoid doing so as the race window is very small (and a problem all over the
> kernel we basically never hit) and locking is rather expensive. In the
> future, we might switch to rcu to handle that more efficiently and avoiding
> these possible races.
> 
> b) PageOffline(): logically offline pages contained in an online memory
> section with a sane memmap. virtio-mem calls these pages "fake offline";
> something like a "temporary" memory hole. The new mechanism I propose will
> be used to handle synchronization as races can be more severe, e.g., when
> reading actual page content here.
> 
> c) Soft offline pages: hwpoisoned pages that are not actually harmful yet,
> but could become harmful in the future. So we better try to remove the page
> from the page allcoator and try to migrate away existing users.
> 
> 
> So page_offline_* handle "b) PageOffline()" only. There is a tiny race
> between pfn_to_online_page(pfn) and looking at the memmap as we have in many
> cases already throughout the kernel, to be tackled in the future.

Right, but here you anyway add locking, so why exclude the first iteration?
 
> (A better name for PageOffline() might make sense; PageSoftOffline() would
> be catchy but interferes with c). PageLogicallyOffline() is ugly;
> PageFakeOffline() might do)
> 
> > > +			/*
> > > +			 * Don't race against drivers that set PageOffline()
> > > +			 * and expect no further page access.
> > > +			 */
> > > +			if (page_offline_frozen == MAX_ORDER_NR_PAGES) {
> > > +				page_offline_unfreeze();
> > > +				page_offline_frozen = 0;
> > > +				cond_resched();
> > > +			}
> > > +			if (!page_offline_frozen++)
> > > +				page_offline_freeze();
> > > +

BTW, did you consider something like

	if (page_offline_frozen++ % MAX_ORDER_NR_PAGES == 0) {
		page_offline_unfreeze();
		cond_resched();
		page_offline_freeze();
	}

We don't seem to care about page_offline_frozen overflows here, do we?

-- 
Sincerely yours,
Mike.