On 06/16/2015 05:15 PM, Andrew Morton wrote: > On Sun, 14 Jun 2015 18:04:43 +0300 Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx> wrote: > >> This patch makes swapin readahead to improve thp collapse rate. >> When khugepaged scanned pages, there can be a few of the pages >> in swap area. >> >> With the patch THP can collapse 4kB pages into a THP when >> there are up to max_ptes_swap swap ptes in a 2MB range. >> >> The patch was tested with a test program that allocates >> 800MB of memory, writes to it, and then sleeps. I force >> the system to swap out all. Afterwards, the test program >> touches the area by writing, it skips a page in each >> 20 pages of the area. >> >> Without the patch, system did not swap in readahead. >> THP rate was %47 of the program of the memory, it >> did not change over time. >> >> With this patch, after 10 minutes of waiting khugepaged had >> collapsed %99 of the program's memory. >> >> ... >> >> +/* >> + * Bring missing pages in from swap, to complete THP collapse. >> + * Only done if khugepaged_scan_pmd believes it is worthwhile. >> + * >> + * Called and returns without pte mapped or spinlocks held, >> + * but with mmap_sem held to protect against vma changes. >> + */ >> + >> +static void __collapse_huge_page_swapin(struct mm_struct *mm, >> + struct vm_area_struct *vma, >> + unsigned long address, pmd_t *pmd, >> + pte_t *pte) >> +{ >> + unsigned long _address; >> + pte_t pteval = *pte; >> + int swap_pte = 0; >> + >> + pte = pte_offset_map(pmd, address); >> + for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE; >> + pte++, _address += PAGE_SIZE) { >> + pteval = *pte; >> + if (is_swap_pte(pteval)) { >> + swap_pte++; >> + do_swap_page(mm, vma, _address, pte, pmd, 0x0, pteval); >> + /* pte is unmapped now, we need to map it */ >> + pte = pte_offset_map(pmd, _address); >> + } >> + } >> + pte--; >> + pte_unmap(pte); >> + trace_mm_collapse_huge_page_swapin(mm, vma->vm_start, swap_pte); >> +} > > This is doing a series of synchronous reads. That will be sloooow on > spinning disks. > > This function should be significantly faster if it first gets all the > necessary I/O underway. I don't think we have a function which exactly > does this. Perhaps generalise swapin_readahead() or open-code > something like Looking at do_swap_page() and __lock_page_or_retry(), I guess there already is a way to do the above. Passing a "flags" of FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT to do_swap_page() should result in do_swap_page() returning with the pte unmapped and the mmap_sem still held if the page was not immediately available to map into the pte (trylock_page succeeds). Ebru, can you try passing the above as the flags argument to do_swap_page(), and see what happens? -- All rights reversed -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>