Re: [PATCH v6 04/16] mm: Tweak readahead loop slightly

John Hubbard
2020-02-18T23:00:16Z

On 2/18/20 2:57 PM, John Hubbard wrote:
> On 2/17/20 10:45 AM, Matthew Wilcox wrote:
>> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>>
>> Eliminate the page_offset variable which was just confusing;
>> record the start of each consecutive run of pages in the
>
>

Darn it, I incorrectly reviewed the N/16 patch, instead of the N/19, for
this one. I thought I had deleted all those! Let me try again with the
correct patch, sorry!!

thanks,
--
John Hubbard
NVIDIA

> OK...presumably for the benefit of a following patch, since it is not
> actually consumed in this patch.
>
>> readahead_control, and move the 'kick off a fresh batch' code to
>> the end of the function for easier use in the next patch.
>
>
> That last bit was actually done in the previous patch, rather than this
> one, right?
>
>>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>> ---
>> mm/readahead.c | 31 +++++++++++++++++++------------
>> 1 file changed, 19 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/readahead.c b/mm/readahead.c
>> index 15329309231f..74791b96013f 100644
>> --- a/mm/readahead.c
>> +++ b/mm/readahead.c
>> @@ -154,7 +154,6 @@ void __do_page_cache_readahead(struct address_space *mapping,
>> unsigned long lookahead_size)
>> {
>> struct inode *inode = mapping->host;
>> - struct page *page;
>> unsigned long end_index; /* The last page we want to read */
>> LIST_HEAD(page_pool);
>> int page_idx;
>> @@ -163,6 +162,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
>> struct readahead_control rac = {
>> .mapping = mapping,
>> .file = filp,
>> + ._start = offset,
>> ._nr_pages = 0,
>> };
>>
>> @@ -175,32 +175,39 @@ void __do_page_cache_readahead(struct address_space *mapping,
>> * Preallocate as many pages as we will need.
>> */
>> for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
>> - pgoff_t page_offset = offset + page_idx;
>
>
> You know...this ends up incrementing offset each time through the
> loop, so yes, the behavior is the same as when using "offset + page_idx".
> However, now it's a little harder to see that.
>
> IMHO the page_offset variable is not actually a bad thing, here. I'd rather
> keep it, all other things being equal (and I don't see any other benefits
> here: line count is the same, for example).
>
> What do you think?
>
>
> thanks,
>