Commit 89b15332 authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds

mm: drop mmap_sem before calling balance_dirty_pages() in write fault

One of our services is observing hanging ps/top/etc under heavy write
IO, and the task states show this is an mmap_sem priority inversion:

A write fault is holding the mmap_sem in read-mode and waiting for
(heavily cgroup-limited) IO in balance_dirty_pages():

    balance_dirty_pages+0x724/0x905
    balance_dirty_pages_ratelimited+0x254/0x390
    fault_dirty_shared_page.isra.96+0x4a/0x90
    do_wp_page+0x33e/0x400
    __handle_mm_fault+0x6f0/0xfa0
    handle_mm_fault+0xe4/0x200
    __do_page_fault+0x22b/0x4a0
    page_fault+0x45/0x50

Somebody tries to change the address space, contending for the mmap_sem in
write-mode:

    call_rwsem_down_write_failed_killable+0x13/0x20
    do_mprotect_pkey+0xa8/0x330
    SyS_mprotect+0xf/0x20
    do_syscall_64+0x5b/0x100
    entry_SYSCALL_64_after_hwframe+0x3d/0xa2

The waiting writer locks out all subsequent readers to avoid lock
starvation, and several threads can be seen hanging like this:

    call_rwsem_down_read_failed+0x14/0x30
    proc_pid_cmdline_read+0xa0/0x480
    __vfs_read+0x23/0x140
    vfs_read+0x87/0x130
    SyS_read+0x42/0x90
    do_syscall_64+0x5b/0x100
    entry_SYSCALL_64_after_hwframe+0x3d/0xa2

To fix this, do what we do for cache read faults already: drop the
mmap_sem before calling into anything IO bound, in this case the
balance_dirty_pages() function, and return VM_FAULT_RETRY.

Link: http://lkml.kernel.org/r/20190924194238.GA29030@cmpxchg.orgSigned-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Reviewed-by: default avatarMatthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1603c8d1
......@@ -2329,27 +2329,6 @@ EXPORT_SYMBOL(generic_file_read_iter);
#ifdef CONFIG_MMU
#define MMAP_LOTSAMISS (100)
static struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
struct file *fpin)
{
int flags = vmf->flags;
if (fpin)
return fpin;
/*
* FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or
* anything, so we only pin the file and drop the mmap_sem if only
* FAULT_FLAG_ALLOW_RETRY is set.
*/
if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) ==
FAULT_FLAG_ALLOW_RETRY) {
fpin = get_file(vmf->vma->vm_file);
up_read(&vmf->vma->vm_mm->mmap_sem);
}
return fpin;
}
/*
* lock_page_maybe_drop_mmap - lock the page, possibly dropping the mmap_sem
* @vmf - the vm_fault for this fault.
......
......@@ -362,6 +362,27 @@ vma_address(struct page *page, struct vm_area_struct *vma)
return max(start, vma->vm_start);
}
static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
struct file *fpin)
{
int flags = vmf->flags;
if (fpin)
return fpin;
/*
* FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or
* anything, so we only pin the file and drop the mmap_sem if only
* FAULT_FLAG_ALLOW_RETRY is set.
*/
if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) ==
FAULT_FLAG_ALLOW_RETRY) {
fpin = get_file(vmf->vma->vm_file);
up_read(&vmf->vma->vm_mm->mmap_sem);
}
return fpin;
}
#else /* !CONFIG_MMU */
static inline void clear_page_mlock(struct page *page) { }
static inline void mlock_vma_page(struct page *page) { }
......
......@@ -2289,10 +2289,11 @@ static vm_fault_t do_page_mkwrite(struct vm_fault *vmf)
*
* The function expects the page to be locked and unlocks it.
*/
static void fault_dirty_shared_page(struct vm_area_struct *vma,
struct page *page)
static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
struct address_space *mapping;
struct page *page = vmf->page;
bool dirtied;
bool page_mkwrite = vma->vm_ops && vma->vm_ops->page_mkwrite;
......@@ -2307,16 +2308,30 @@ static void fault_dirty_shared_page(struct vm_area_struct *vma,
mapping = page_rmapping(page);
unlock_page(page);
if (!page_mkwrite)
file_update_time(vma->vm_file);
/*
* Throttle page dirtying rate down to writeback speed.
*
* mapping may be NULL here because some device drivers do not
* set page.mapping but still dirty their pages
*
* Drop the mmap_sem before waiting on IO, if we can. The file
* is pinning the mapping, as per above.
*/
if ((dirtied || page_mkwrite) && mapping) {
/*
* Some device drivers do not set page.mapping
* but still dirty their pages
*/
struct file *fpin;
fpin = maybe_unlock_mmap_for_io(vmf, NULL);
balance_dirty_pages_ratelimited(mapping);
if (fpin) {
fput(fpin);
return VM_FAULT_RETRY;
}
}
if (!page_mkwrite)
file_update_time(vma->vm_file);
return 0;
}
/*
......@@ -2571,6 +2586,7 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
__releases(vmf->ptl)
{
struct vm_area_struct *vma = vmf->vma;
vm_fault_t ret = VM_FAULT_WRITE;
get_page(vmf->page);
......@@ -2594,10 +2610,10 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
wp_page_reuse(vmf);
lock_page(vmf->page);
}
fault_dirty_shared_page(vma, vmf->page);
ret |= fault_dirty_shared_page(vmf);
put_page(vmf->page);
return VM_FAULT_WRITE;
return ret;
}
/*
......@@ -3641,7 +3657,7 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
return ret;
}
fault_dirty_shared_page(vma, vmf->page);
ret |= fault_dirty_shared_page(vmf);
return ret;
}
......
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