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SubjectRe: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS
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> On Jul 10, 2019, at 11:48 AM, Johannes Weiner <hannes@cmpxchg.org> 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@surriel.com>
>> Signed-off-by: Song Liu <songliubraving@fb.com>
>
> 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

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