On 2024/2/2 17:02, Liu Shixin wrote: > > On 2024/2/2 1:31, Jan Kara wrote: >> On Thu 01-02-24 18:41:30, Liu Shixin wrote: >>> On 2024/2/1 17:37, Jan Kara wrote: >>>> On Thu 01-02-24 18:08:35, Liu Shixin wrote: >>>>> When the pagefault is not for write and the refault distance is close, >>>>> the page will be activated directly. If there are too many such pages in >>>>> a file, that means the pages may be reclaimed immediately. >>>>> In such situation, there is no positive effect to read-ahead since it will >>>>> only waste IO. So collect the number of such pages and when the number is >>>>> too large, stop bothering with read-ahead for a while until it decreased >>>>> automatically. >>>>> >>>>> Define 'too large' as 10000 experientially, which can solves the problem >>>>> and does not affect by the occasional active refault. >>>>> >>>>> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx> >>>> So I'm not convinced this new logic is needed. We already have >>>> ra->mmap_miss which gets incremented when a page fault has to read the page >>>> (and decremented when a page fault found the page already in cache). This >>>> should already work to detect trashing as well, shouldn't it? If it does >>>> not, why? >>>> >>>> Honza >>> ra->mmap_miss doesn't help, it increased only one in do_sync_mmap_readahead() >>> and then decreased one for every page in filemap_map_pages(). So in this scenario, >>> it can't exceed MMAP_LOTSAMISS. >> I see, OK. But that's a (longstanding) bug in how mmap_miss is handled. Can >> you please test whether attached patches fix the trashing for you? At least >> now I can see mmap_miss properly increments when we are hitting uncached >> pages... Thanks! >> >> Honza > The patch doesn't seem to have much effect. I will try to analyze why it doesn't work. > The attached file is my testcase. > > Thanks, I think I figured out why mmap_miss doesn't work. After do_sync_mmap_readahead(), there is a __filemap_get_folio() to make sure the page is ready. Then, it is ready too in filemap_map_pages(), so the mmap_miss will decreased once. mmap_miss goes back to 0, and can't stop read-ahead. Overall, I don't think mmap_miss can solve this problem. .
