Re: Buffered I/O broken on s390x with page faults disabled (gfs2)

[David Hildenbrand <david@redhat.com>]

On 08.03.22 09:21, David Hildenbrand wrote:
> On 08.03.22 00:18, Linus Torvalds wrote:
>> On Mon, Mar 7, 2022 at 2:52 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>>>
>>> After generic_file_read_iter() returns a short or empty read, we fault
>>> in some pages with fault_in_iov_iter_writeable(). This succeeds, but
>>> the next call to generic_file_read_iter() returns -EFAULT and we're
>>> not making any progress.
>>
>> Since this is s390-specific, I get the very strong feeling that the
>>
>>   fault_in_iov_iter_writeable ->
>>     fault_in_safe_writeable ->
>>       __get_user_pages_locked ->
>>         __get_user_pages
>>
>> path somehow successfully finds the page, despite it not being
>> properly accessible in the page tables.
> 
> As raised offline already, I suspect
> 
> shrink_active_list()
> ->page_referenced()
>  ->page_referenced_one()
>   ->ptep_clear_flush_young_notify()
>    ->ptep_clear_flush_young()
> 
> which results on s390x in:
> 
> static inline pte_t pte_mkold(pte_t pte)
> {
> 	pte_val(pte) &= ~_PAGE_YOUNG;
> 	pte_val(pte) |= _PAGE_INVALID;
> 	return pte;
> }
> 
> static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> 					    unsigned long addr, pte_t *ptep)
> {
> 	pte_t pte = *ptep;
> 
> 	pte = ptep_xchg_direct(vma->vm_mm, addr, ptep, pte_mkold(pte));
> 	return pte_young(pte);
> }
> 
> 
> _PAGE_INVALID is the actual HW bit, _PAGE_PRESENT is a
> pure SW bit. AFAIU, pte_present() still holds:
> 
> static inline int pte_present(pte_t pte)
> {
> 	/* Bit pattern: (pte & 0x001) == 0x001 */
> 	return (pte_val(pte) & _PAGE_PRESENT) != 0;
> }
> 
> 
> pte_mkyoung() will revert that action:
> 
> static inline pte_t pte_mkyoung(pte_t pte)
> {
> 	pte_val(pte) |= _PAGE_YOUNG;
> 	if (pte_val(pte) & _PAGE_READ)
> 		pte_val(pte) &= ~_PAGE_INVALID;
> 	return pte;
> }
> 
> 
> and pte_modify() will adjust it properly again:
> 
> /*
>  * The following pte modification functions only work if
>  * pte_present() is true. Undefined behaviour if not..
>  */
> static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> {
> 	pte_val(pte) &= _PAGE_CHG_MASK;
> 	pte_val(pte) |= pgprot_val(newprot);
> 	/*
> 	 * newprot for PAGE_NONE, PAGE_RO, PAGE_RX, PAGE_RW and PAGE_RWX
> 	 * has the invalid bit set, clear it again for readable, young pages
> 	 */
> 	if ((pte_val(pte) & _PAGE_YOUNG) && (pte_val(pte) & _PAGE_READ))
> 		pte_val(pte) &= ~_PAGE_INVALID;
> 	/*
> 	 * newprot for PAGE_RO, PAGE_RX, PAGE_RW and PAGE_RWX has the page
> 	 * protection bit set, clear it again for writable, dirty pages
> 	 */
> 	if ((pte_val(pte) & _PAGE_DIRTY) && (pte_val(pte) & _PAGE_WRITE))
> 		pte_val(pte) &= ~_PAGE_PROTECT;
> 	return pte;
> }
> 
> 
> 
> Which leaves me wondering if there is a way in GUP whereby
> we would lookup that page and not clear _PAGE_INVALID,
> resulting in GUP succeeding but faults via the MMU still
> faulting on _PAGE_INVALID.


follow_page_pte() has this piece of code:

	if (flags & FOLL_TOUCH) {
		if ((flags & FOLL_WRITE) &&
		    !pte_dirty(pte) && !PageDirty(page))
			set_page_dirty(page);
		/*
		 * pte_mkyoung() would be more correct here, but atomic care
		 * is needed to avoid losing the dirty bit: it is easier to use
		 * mark_page_accessed().
		 */
		mark_page_accessed(page);
	}

Which at least to me suggests that, although the page is marked accessed and GUP
succeeds, that the PTE might still have _PAGE_INVALID set after we succeeded GUP.


On s390x, there is no HW dirty bit, so we might just be able to do a proper
pte_mkyoung() here instead of the mark_page_accessed().

-- 
Thanks,

David / dhildenb