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

[Alexander Duyck]

On Thu, 2020-02-20 at 22:35 +0000, Mel Gorman wrote:
> 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.

The comment I can do, that shouldn't be an issue. The point I was getting
at is that a separate drain call is expected for this any time the
function is not returning an error, and the only way it can return an
error is if there was a reporting issue.

> > > 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.

I would think adding that would confuse things even more. There is a break
statement at the end of the loop that will break out if err is set. So we
should never hit the VM_WARN_ON_ONCE because err should always be 0 before
we even attempt to isolate the page. I think something like the following
would probably make more sense:

        err = page_reporting_cycle(prdev, zone, order, mt,
                                   sgl, &offset);
        if (err) {
                /*
                 * We should have drained the scatterlist
                 * prior to exiting page_reporting_cycle if
                 * we encountered an error. If we did not
                 * then this could result in a memory leak.
                 * Verify that the end of the scatterlist
                 * was cleared prior to us getting here.
                 */
                sgl = &sgl[PAGE_REPORTING_CAPACITY - 1];
                VM_WARN_ON_ONCE(sg_page(sgl));
                return err;
        }

With that we are more-or-less making certain that they called
page_reporting_drain which will zero the scatterlist.