Re: [PATCH v17 4/9] mm: Introduce Reported pages

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Feb 20, 2020 at 10:44:21AM -0800, Alexander Duyck wrote:
> > > +static int
> > > +page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
> > > +		     unsigned int order, unsigned int mt,
> > > +		     struct scatterlist *sgl, unsigned int *offset)
> > > +{
> > > +	struct free_area *area = &zone->free_area[order];
> > > +	struct list_head *list = &area->free_list[mt];
> > > +	unsigned int page_len = PAGE_SIZE << order;
> > > +	struct page *page, *next;
> > > +	int err = 0;
> > > +
> > > +	/*
> > > +	 * Perform early check, if free area is empty there is
> > > +	 * nothing to process so we can skip this free_list.
> > > +	 */
> > > +	if (list_empty(list))
> > > +		return err;
> > > +
> > > +	spin_lock_irq(&zone->lock);
> > > +
> > > +	/* loop through free list adding unreported pages to sg list */
> > > +	list_for_each_entry_safe(page, next, list, lru) {
> > > +		/* We are going to skip over the reported pages. */
> > > +		if (PageReported(page))
> > > +			continue;
> > > +
> > > +		/* Attempt to pull page from list */
> > > +		if (!__isolate_free_page(page, order))
> > > +			break;
> > > +
> > 
> > Might want to note that you are breaking because the only reason to fail
> > the isolation is that watermarks are not met and we are likely under
> > memory pressure. It's not a big issue.
> > 
> > However, while I think this is correct, it's hard to follow. This loop can
> > be broken out of with pages still on the scatter gather list. The current
> > flow guarantees that err will not be set at this point so the caller
> > cleans it up so we always drain the list either here or in the caller.
> 
> I can probably submit a follow-up patch to update the comments. The reason
> for not returning an error is because I didn't consider it an error that
> we encountered the watermark and were not able to pull any more pages.
> Instead I considered that the "stop" point for this pass and have it just
> exit out of the loop and flush the data.
> 

I don't consider it an error and I don't think you should return an
error. The comment just needs to explain that the draining happens in
the caller in this case. That should be enough of a warning to a future
developer to double check the flow after any changes to make sure the
drain is reached.

> > While I think it works, it's a bit fragile. I recommend putting a comment
> > above this noting why it's safe and put a VM_WARN_ON_ONCE(err) before the
> > break in case someone tries to change this in a years time and does not
> > spot that the flow to reach page_reporting_drain *somewhere* is critical.
> 
> I assume this isn't about this section, but the section below?
> 

I meant something like

if (!__isolate_free_page(page, order)) {
	VM_WARN_ON_ONCE(err);
	break;
}

Because at this point it's possible there are entries that should go
through page_reporting_drain() but the caller will not call
page_reporting_drain() in the event of an error.

-- 
Mel Gorman
SUSE Labs