Re: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS

[Song Liu]

> On Jun 24, 2019, at 5:47 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> 
> On Sat, Jun 22, 2019 at 10:47:48PM -0700, Song Liu wrote:
>> This patch is (hopefully) the first step to enable THP for non-shmem
>> filesystems.
>> 
>> This patch enables an application to put part of its text sections to THP
>> via madvise, for example:
>> 
>>    madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
>> 
>> We tried to reuse the logic for THP on tmpfs.
>> 
>> Currently, write is not supported for non-shmem THP. khugepaged will only
>> process vma with VM_DENYWRITE. The next patch will handle writes, which
>> would only happen when the vma with VM_DENYWRITE is unmapped.
>> 
>> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
>> feature.
>> 
>> Acked-by: Rik van Riel <riel@xxxxxxxxxxx>
>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>> ---
>> mm/Kconfig      | 11 ++++++
>> mm/filemap.c    |  4 +--
>> mm/khugepaged.c | 90 ++++++++++++++++++++++++++++++++++++++++---------
>> mm/rmap.c       | 12 ++++---
>> 4 files changed, 96 insertions(+), 21 deletions(-)
>> 
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index f0c76ba47695..0a8fd589406d 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -762,6 +762,17 @@ config GUP_BENCHMARK
>> 
>> 	  See tools/testing/selftests/vm/gup_benchmark.c
>> 
>> +config READ_ONLY_THP_FOR_FS
>> +	bool "Read-only THP for filesystems (EXPERIMENTAL)"
>> +	depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
>> +
>> +	help
>> +	  Allow khugepaged to put read-only file-backed pages in THP.
>> +
>> +	  This is marked experimental because it is a new feature. Write
>> +	  support of file THPs will be developed in the next few release
>> +	  cycles.
>> +
>> config ARCH_HAS_PTE_SPECIAL
>> 	bool
>> 
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 5f072a113535..e79ceccdc6df 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -203,8 +203,8 @@ static void unaccount_page_cache_page(struct address_space *mapping,
>> 		__mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);
>> 		if (PageTransHuge(page))
>> 			__dec_node_page_state(page, NR_SHMEM_THPS);
>> -	} else {
>> -		VM_BUG_ON_PAGE(PageTransHuge(page), page);
>> +	} else if (PageTransHuge(page)) {
>> +		__dec_node_page_state(page, NR_FILE_THPS);
>> 	}
>> 
>> 	/*
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 158cad542627..090127e4e185 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -48,6 +48,7 @@ enum scan_result {
>> 	SCAN_CGROUP_CHARGE_FAIL,
>> 	SCAN_EXCEED_SWAP_PTE,
>> 	SCAN_TRUNCATED,
>> +	SCAN_PAGE_HAS_PRIVATE,
>> };
>> 
>> #define CREATE_TRACE_POINTS
>> @@ -404,7 +405,11 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
>> 	    (vm_flags & VM_NOHUGEPAGE) ||
>> 	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> 		return false;
>> -	if (shmem_file(vma->vm_file)) {
>> +
>> +	if (shmem_file(vma->vm_file) ||
>> +	    (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>> +	     vma->vm_file &&
>> +	     (vm_flags & VM_DENYWRITE))) {
>> 		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
>> 			return false;
>> 		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
>> @@ -456,8 +461,9 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>> 	unsigned long hstart, hend;
>> 
>> 	/*
>> -	 * khugepaged does not yet work on non-shmem files or special
>> -	 * mappings. And file-private shmem THP is not supported.
>> +	 * khugepaged only supports read-only files for non-shmem files.
>> +	 * khugepaged does not yet work on special mappings. And
>> +	 * file-private shmem THP is not supported.
>> 	 */
>> 	if (!hugepage_vma_check(vma, vm_flags))
>> 		return 0;
>> @@ -1287,12 +1293,12 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>> }
>> 
>> /**
>> - * collapse_file - collapse small tmpfs/shmem pages into huge one.
>> + * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.
>>  *
>>  * Basic scheme is simple, details are more complex:
>>  *  - allocate and lock a new huge page;
>>  *  - scan page cache replacing old pages with the new one
>> - *    + swap in pages if necessary;
>> + *    + swap/gup in pages if necessary;
>>  *    + fill in gaps;
>>  *    + keep old pages around in case rollback is required;
>>  *  - if replacing succeeds:
>> @@ -1316,7 +1322,11 @@ static void collapse_file(struct mm_struct *mm,
>> 	LIST_HEAD(pagelist);
>> 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
>> 	int nr_none = 0, result = SCAN_SUCCEED;
>> +	bool is_shmem = shmem_file(file);
>> 
>> +#ifndef CONFIG_READ_ONLY_THP_FOR_FS
>> +	VM_BUG_ON(!is_shmem);
>> +#endif
> 
> 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);

Will fix. 

> 
>> 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
>> 
>> 	/* Only allocate from the target node */
>> @@ -1348,7 +1358,8 @@ static void collapse_file(struct mm_struct *mm,
>> 	} while (1);
>> 
>> 	__SetPageLocked(new_page);
>> -	__SetPageSwapBacked(new_page);
>> +	if (is_shmem)
>> +		__SetPageSwapBacked(new_page);
>> 	new_page->index = start;
>> 	new_page->mapping = mapping;
>> 
>> @@ -1363,7 +1374,7 @@ static void collapse_file(struct mm_struct *mm,
>> 		struct page *page = xas_next(&xas);
>> 
>> 		VM_BUG_ON(index != xas.xa_index);
>> -		if (!page) {
>> +		if (is_shmem && !page) {
>> 			/*
>> 			 * Stop if extent has been truncated or hole-punched,
>> 			 * and is now completely empty.
>> @@ -1384,7 +1395,7 @@ static void collapse_file(struct mm_struct *mm,
>> 			continue;
>> 		}
>> 
>> -		if (xa_is_value(page) || !PageUptodate(page)) {
>> +		if (is_shmem && (xa_is_value(page) || !PageUptodate(page))) {
>> 			xas_unlock_irq(&xas);
>> 			/* swap in or instantiate fallocated page */
>> 			if (shmem_getpage(mapping->host, index, &page,
>> @@ -1392,6 +1403,23 @@ static void collapse_file(struct mm_struct *mm,
>> 				result = SCAN_FAIL;
>> 				goto xa_unlocked;
>> 			}
>> +		} else if (!page || xa_is_value(page)) {
>> +			xas_unlock_irq(&xas);
>> +			page_cache_sync_readahead(mapping, &file->f_ra, file,
>> +						  index, PAGE_SIZE);
>> +			lru_add_drain();
> 
> Why?

isolate_lru_page() is likely to fail if we don't drain the pagevecs. 

> 
>> +			page = find_lock_page(mapping, index);
>> +			if (unlikely(page == NULL)) {
>> +				result = SCAN_FAIL;
>> +				goto xa_unlocked;
>> +			}
>> +		} else if (!PageUptodate(page)) {
> 
> Maybe we should try wait_on_page_locked() here before give up?

Are you referring to the "if (!PageUptodate(page))" case? 

> 
>> +			VM_BUG_ON(is_shmem);
>> +			result = SCAN_FAIL;
>> +			goto xa_locked;
>> +		} else if (!is_shmem && PageDirty(page)) {
>> +			result = SCAN_FAIL;
>> +			goto xa_locked;
>> 		} else if (trylock_page(page)) {
>> 			get_page(page);
>> 			xas_unlock_irq(&xas);
> -- 
> Kirill A. Shutemov