[LU-18449] Handling kernel mmap read-ahead triggering by advise(MADV_HUGEPAGE)

• Type: Bug
• Resolution: Unresolved
• Priority: Minor
• Severity: 3
• Assignee: Qian Yingjin
• Reporter: Qian Yingjin

Description

kernel:LustreError: 126979:0:(rw.c:2162:ll_readpage()) ASSERTION( !ra_assert ) failed: 

Found a possible bug in kernel to trigger readahead in Linux kernel:

static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
{
	struct file *file = vmf->vma->vm_file;
	struct file_ra_state *ra = &file->f_ra;
	struct address_space *mapping = file->f_mapping;
	DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
	struct file *fpin = NULL;
	unsigned long vm_flags = vmf->vma->vm_flags;
	unsigned int mmap_miss;

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
	/* Use the readahead code, even if readahead is disabled */
	if (vm_flags & VM_HUGEPAGE) {
		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
		ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
		ra->size = HPAGE_PMD_NR;
		/*
		 * Fetch two PMD folios, so we get the chance to actually
		 * readahead, unless we've been told not to.
		 */
		if (!(vm_flags & VM_RAND_READ))
			ra->size *= 2;
		ra->async_size = HPAGE_PMD_NR;
		page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
		return fpin;
	}
#endif

	/* If we don't want any read-ahead, don't bother */
	if (vm_flags & VM_RAND_READ)
		return fpin;
	if (!ra->ra_pages)
		return fpin;
        ...
}

Usually the kernel readahead will disabled by setting ra_pages with 0.
However,
when CONFIG_TRANSPARENT_HUGEPAGE is enabled and vm flagged with VM_HUGEPAGE, it will trigger the kernel readahead.
When the kernel is configed with CONFIG_TRANSPARENT_HUGEPAGE and the application is set VM_HUGEPAGE via MADV_HUGEPAGE via madvise() system call, it is possible to trigger these kind of read-ahead from Linux kernel and out the control of Lustre.