Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS

[Song Liu]

> On Jul 10, 2019, at 11:48 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> 
> On Mon, Jun 24, 2019 at 05:12:45PM -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. sys_mmap() ignores VM_DENYWRITE requests
>> (see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is
>> execve(). This requirement limits non-shmem THP to text sections.
>> 
>> The next patch will handle writes, which would only happen when the all
>> the vmas with VM_DENYWRITE are 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>
> 
> This is really cool, and less invasive than I anticipated. Nice work.
> 
> I only have one concern and one question:
> 
>> @@ -1392,6 +1401,29 @@ 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);
>> +			/* drain pagevecs to help isolate_lru_page() */
>> +			lru_add_drain();
>> +			page = find_lock_page(mapping, index);
>> +			if (unlikely(page == NULL)) {
>> +				result = SCAN_FAIL;
>> +				goto xa_unlocked;
>> +			}
>> +		} else if (!PageUptodate(page)) {
>> +			VM_BUG_ON(is_shmem);
>> +			xas_unlock_irq(&xas);
>> +			wait_on_page_locked(page);
>> +			if (!trylock_page(page)) {
>> +				result = SCAN_PAGE_LOCK;
>> +				goto xa_unlocked;
>> +			}
>> +			get_page(page);
>> +		} else if (!is_shmem && PageDirty(page)) {
>> +			result = SCAN_FAIL;
>> +			goto xa_locked;
>> 		} else if (trylock_page(page)) {
>> 			get_page(page);
>> 			xas_unlock_irq(&xas);
> 
> The many else ifs here check fairly complex page state and are hard to
> follow and verify mentally. In fact, it's a bit easier now in the
> patch when you see how it *used* to work with just shmem, but the end
> result is fragile from a maintenance POV.
> 
> The shmem and file cases have little in common - basically only the
> trylock_page(). Can you please make one big 'if (is_shmem) {} {}'
> structure instead that keeps those two scenarios separate?

Good point! Will fix in next version. 

> 
>> @@ -1426,6 +1458,12 @@ static void collapse_file(struct mm_struct *mm,
>> 			goto out_unlock;
>> 		}
>> 
>> +		if (page_has_private(page) &&
>> +		    !try_to_release_page(page, GFP_KERNEL)) {
>> +			result = SCAN_PAGE_HAS_PRIVATE;
>> +			break;
>> +		}
>> +
>> 		if (page_mapped(page))
>> 			unmap_mapping_pages(mapping, index, 1, false);
> 
>> @@ -1607,6 +1658,17 @@ static void khugepaged_scan_file(struct mm_struct *mm,
>> 			break;
>> 		}
>> 
>> +		if (page_has_private(page) && trylock_page(page)) {
>> +			int ret;
>> +
>> +			ret = try_to_release_page(page, GFP_KERNEL);
>> +			unlock_page(page);
>> +			if (!ret) {
>> +				result = SCAN_PAGE_HAS_PRIVATE;
>> +				break;
>> +			}
>> +		}
>> +
>> 		if (page_count(page) != 1 + page_mapcount(page)) {
>> 			result = SCAN_PAGE_COUNT;
>> 			break;
> 
> There is already a try_to_release() inside the page lock section in
> collapse_file(). I'm assuming you added this one because private data
> affects the refcount. But it seems a bit overkill just for that; we
> could also still fail the check, in which case we'd have dropped the
> buffers in vain. Can you fix the check instead?
> 
> There is an is_page_cache_freeable() function in vmscan.c that handles
> private fs references:
> 
> static inline int is_page_cache_freeable(struct page *page)
> {
> 	/*
> 	 * A freeable page cache page is referenced only by the caller
> 	 * that isolated the page, the page cache and optional buffer
> 	 * heads at page->private.
> 	 */
> 	int page_cache_pins = PageTransHuge(page) && PageSwapCache(page) ?
> 		HPAGE_PMD_NR : 1;
> 	return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
> }
> 
> Wouldn't this work here as well?

Good point! Let me try fix this. 

Thanks,
Song