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Date: Sat, 12 Sep 2020 23:32:07 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Amir Goldstein <amir73il@...il.com>,
Hugh Dickins <hughd@...gle.com>,
Michael Larabel <Michael@...haellarabel.com>,
Ted Ts'o <tytso@...gle.com>,
Andreas Dilger <adilger.kernel@...ger.ca>,
Ext4 Developers List <linux-ext4@...r.kernel.org>,
Jan Kara <jack@...e.cz>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernel Benchmarking
On Sat, Sep 12, 2020 at 10:59:40AM -0700, Linus Torvalds wrote:
> Anyway, I don't have a great solution. I have a few options (roughly
> ordered by "simplest to most complex"):
>
> (a) just revert
> (b) add some busy-spinning
> (c) reader-writer page lock
> (d) try to de-emphasize the page lock
>
> Option (d) is "we already have a locking in many filesystems that give
> us exclusion between faulting in a page, and the truncate/hole punch,
> so we shouldn't use the page lock at all".
>
> I do think that the locking that filesystems do is in many ways
> inferior - it's done on a per-inode basis rather than on a per-page
> basis. But if the filesystems end up doing that *anyway*, what's the
> advantage of the finer granularity one? And *because* the common case
> is all about the reading case, the bigger granularity tends to work
> very well in practice, and basically never sees contention.
I guess this is option (e). Completely untested; not even compiled,
but it might be a design that means filesystems don't need to take
per-inode locks. I probably screwed up the drop-mmap-lock-for-io
parts of filemap_fault. I definitely didn't update DAX for the
new parameter for finish_fault(), and now I think about it, I didn't
update the header file either, so it definitely won't compile.
diff --git a/mm/filemap.c b/mm/filemap.c
index 1aaea26556cc..3909613f1c9c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2602,8 +2602,22 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
}
}
+ if (fpin)
+ goto out_retry;
+ if (likely(PageUptodate(page)))
+ goto uptodate;
+
if (!lock_page_maybe_drop_mmap(vmf, page, &fpin))
goto out_retry;
+ VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
+
+ /* Did somebody else update it for us? */
+ if (PageUptodate(page)) {
+ unlock_page(page);
+ if (fpin)
+ goto out_retry;
+ goto uptodate;
+ }
/* Did it get truncated? */
if (unlikely(compound_head(page)->mapping != mapping)) {
@@ -2611,14 +2625,6 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
put_page(page);
goto retry_find;
}
- VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
-
- /*
- * We have a locked page in the page cache, now we need to check
- * that it's up-to-date. If not, it is going to be due to an error.
- */
- if (unlikely(!PageUptodate(page)))
- goto page_not_uptodate;
/*
* We've made it this far and we had to drop our mmap_lock, now is the
@@ -2641,10 +2647,6 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
return VM_FAULT_SIGBUS;
}
- vmf->page = page;
- return ret | VM_FAULT_LOCKED;
-
-page_not_uptodate:
/*
* Umm, take care of errors if the page isn't up-to-date.
* Try to re-read it _once_. We do this synchronously,
@@ -2680,6 +2682,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
if (fpin)
fput(fpin);
return ret | VM_FAULT_RETRY;
+
+uptodate:
+ vmf->page = page;
+ return ret | VM_FAULT_UPTODATE;
}
EXPORT_SYMBOL(filemap_fault);
diff --git a/mm/memory.c b/mm/memory.c
index 469af373ae76..48fb04e75a3a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3460,6 +3460,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
return VM_FAULT_HWPOISON;
}
+ if (ret & VM_FAULT_UPTODATE)
+ return ret;
if (unlikely(!(ret & VM_FAULT_LOCKED)))
lock_page(vmf->page);
else
@@ -3684,7 +3686,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct page *page)
*
* Return: %0 on success, %VM_FAULT_ code in case of error.
*/
-vm_fault_t finish_fault(struct vm_fault *vmf)
+vm_fault_t finish_fault(struct vm_fault *vmf, vm_fault_t ret2)
{
struct page *page;
vm_fault_t ret = 0;
@@ -3704,9 +3706,17 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
ret = check_stable_address_space(vmf->vma->vm_mm);
if (!ret)
ret = alloc_set_pte(vmf, page);
+ if (ret2 & VM_FAULT_UPTODATE) {
+ if (!PageUptodate(page)) {
+ /* probably other things to do here */
+ page_remove_rmap(page);
+ pte_clear(vmf->vma->vm_mm, vmf->address, vmf->pte);
+ put_page(page);
+ }
+ }
if (vmf->pte)
pte_unmap_unlock(vmf->pte, vmf->ptl);
- return ret;
+ return ret | ret2;
}
static unsigned long fault_around_bytes __read_mostly =
@@ -3844,8 +3854,9 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
return ret;
- ret |= finish_fault(vmf);
- unlock_page(vmf->page);
+ ret = finish_fault(vmf, ret);
+ if (!(ret & VM_FAULT_UPTODATE))
+ unlock_page(vmf->page);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
put_page(vmf->page);
return ret;
@@ -3878,8 +3889,9 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
copy_user_highpage(vmf->cow_page, vmf->page, vmf->address, vma);
__SetPageUptodate(vmf->cow_page);
- ret |= finish_fault(vmf);
- unlock_page(vmf->page);
+ ret = finish_fault(vmf, ret);
+ if (!(ret & VM_FAULT_UPTODATE))
+ unlock_page(vmf->page);
put_page(vmf->page);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
goto uncharge_out;
@@ -3912,10 +3924,11 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
}
}
- ret |= finish_fault(vmf);
+ ret = finish_fault(vmf, ret);
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE |
VM_FAULT_RETRY))) {
- unlock_page(vmf->page);
+ if (!(ret & VM_FAULT_UPTODATE))
+ unlock_page(vmf->page);
put_page(vmf->page);
return ret;
}
diff --git a/mm/truncate.c b/mm/truncate.c
index dd9ebc1da356..649381703f31 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -176,6 +176,7 @@ void do_invalidatepage(struct page *page, unsigned int offset,
static void
truncate_cleanup_page(struct address_space *mapping, struct page *page)
{
+ ClearPageUptodate(page);
if (page_mapped(page)) {
pgoff_t nr = PageTransHuge(page) ? HPAGE_PMD_NR : 1;
unmap_mapping_pages(mapping, page->index, nr, false);
@@ -738,7 +739,6 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
1, false);
}
}
- BUG_ON(page_mapped(page));
ret2 = do_launder_page(mapping, page);
if (ret2 == 0) {
if (!invalidate_complete_page2(mapping, page))
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