On 2/20/20 7:43 PM, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 07:19:58PM -0800, John Hubbard wrote:
>>> +static inline struct page *readahead_page(struct readahead_control *rac)
>>> +{
>>> + struct page *page;
>>> +
>>> + BUG_ON(rac->_batch_count > rac->_nr_pages);
>>> + rac->_nr_pages -= rac->_batch_count;
>>> + rac->_index += rac->_batch_count;
>>> + rac->_batch_count = 0;
>>
>>
>> Is it intentional, to set rac->_batch_count twice (here, and below)? The
>> only reason I can see is if a caller needs to use ->_batch_count in the
>> "return NULL" case, which doesn't seem to come up...
>
> Ah, but it does. Not in this patch, but the next one ...
>
> + if (aops->readahead) {
> + aops->readahead(rac);
> + /* Clean up the remaining pages */
> + while ((page = readahead_page(rac))) {
> + unlock_page(page);
> + put_page(page);
> + }
>
> In the normal case, the ->readahead method will consume all the pages,
> and we need readahead_page() to do nothing if it is called again.
>
>>> + if (!rac->_nr_pages)
>>> + return NULL;
>
> ... admittedly I could do:
>
> if (!rac->_nr_pages) {
> rac->_batch_count = 0;
> return NULL;
> }
>
> which might be less confusing.
Yes, that would be a nice bit of polish if you end up doing another revision for other
reasons.
>
>>> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
>>> readahead_count(rac));
>>> /* Clean up the remaining pages */
>>> put_pages_list(pages);
>>> - goto out;
>>> - }
>>> -
>>> - for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
>>> - struct page *page = lru_to_page(pages);
>>> - list_del(&page->lru);
>>> - if (!add_to_page_cache_lru(page, rac->mapping, page->index,
>>> - gfp))
>>> + rac->_index += rac->_nr_pages;
>>> + rac->_nr_pages = 0;
>>> + } else {
>>> + while ((page = readahead_page(rac))) {
>>> aops->readpage(rac->file, page);
>>> - put_page(page);
>>> + put_page(page);
>>> + }
>>> }
>>>
>>> -out:
>>> blk_finish_plug(&plug);
>>>
>>> BUG_ON(!list_empty(pages));
>>> - rac->_nr_pages = 0;
>>> + BUG_ON(readahead_count(rac));
>>> +
>>> +out:
>>> + /* If we were called due to a conflicting page, skip over it */
>>
>> Tiny documentation nit: What if we were *not* called due to a conflicting page?
>> (And what is a "conflicting page", in this context, btw?) The next line unconditionally
>> moves the index ahead, so the "if" part of the comment really confuses me.
>
> By the end of the series, read_pages() is called in three places:
>
> 1. if (page && !xa_is_value(page)) {
> read_pages(&rac, &page_pool);
>
> 2. } else if (add_to_page_cache_lru(page, mapping, index + i,
> gfp_mask) < 0) {
> put_page(page);
> read_pages(&rac, &page_pool);
>
> 3. read_pages(&rac, &page_pool);
>
> In the first two cases, there's an existing page in the page cache
> (which conflicts with this readahead operation), and so we need to
> advance index. In the third case, we're exiting the function, so it
> does no harm to advance index one further.
OK, I see. As you know, I tend toward maybe over-documenting, but what about
adding just a *few* hints to help new readers, like this approximately (maybe
it should be pared down):
diff --git a/mm/readahead.c b/mm/readahead.c
index 9fb5f77dcf69..0dd5b09c376e 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -114,6 +114,10 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
EXPORT_SYMBOL(read_cache_pages);
+/*
+ * Read pages into the page cache, OR skip over a page if it is already in the
+ * page cache.
+ */
static void read_pages(struct readahead_control *rac, struct list_head *pages)
{
const struct address_space_operations *aops = rac->mapping->a_ops;
@@ -152,7 +156,11 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages)
BUG_ON(readahead_count(rac));
out:
- /* If we were called due to a conflicting page, skip over it */
+ /*
+ * This routine might have been called in order to skip over a page
+ * that is already in the page cache. And for other cases, the index is
+ * ignored by the caller. So just increment unconditionally:
+ */
rac->_index++;
}
?
>
>>> + } else if (add_to_page_cache_lru(page, mapping, index + i,
>>> + gfp_mask) < 0) {
>>
>> I still think you'll want to compare against !=0, rather than < 0, here.
>
> I tend to prefer < 0 when checking for an error value in case the function
> decides to start using positive numbers to mean something. I don't think
> it's a particularly important preference though (after all, returning 1
> might mean "failed, but for this weird reason rather than an errno").
>
>>> + put_page(page);
>>> + read_pages(&rac, &page_pool);
>>
>> Doing a read_pages() in the error case is because...actually, I'm not sure yet.
>> Why do we do this? Effectively it's a retry?
>
> Same as the reason we call read_pages() if we found a page in the page
> cache earlier -- we're sending down a set of pages which are consecutive
> in the file's address space, and now we have to skip one. At least one ;-)
>
Got it. Finally. :)
thanks,
--
John Hubbard
NVIDIA
