On Thu, Jun 30, 2016 at 11:26:45AM +0530, Anshuman Khandual wrote:
<snip>
Did you get a chance to test the driver out ? I am still concerned about how to handle the struct address_space override problem within the struct page.
Hi Anshuman,
Slow but I am working on that. :) However, as I said, I want to do it
I really appreciate. Was just curious about the problem and any potential solution we can look into.
after soft landing of current non-lru-no-mapped page migration to solve current real field issues.
yeah it makes sense.
About the overriding problem of non-lru-mapped-page, I implemented dummy driver as miscellaneous device and in test_mmap(file_operations.mmap), I changed a_ops with my address_space_operations.
int test_mmap(struct file *filp, struct vm_area_struct *vma) { filp->f_mapping->a_ops = &test_aops; vma->vm_ops = &test_vm_ops; vma->vm_private_data = filp->private_data; return 0; }
Okay.
test_aops should have *set_page_dirty* overriding.
static int test_set_pag_dirty(struct page *page) { if (!PageDirty(page)) SetPageDirty*page); return 0; }
Otherwise, it goes BUG_ON during radix tree operation because currently try_to_unmap is designed for file-lru pages which lives in page cache so it propagates page table dirty bit to PG_dirty flag of struct page by set_page_dirty. And set_page_dirty want to mark dirty tag in radix tree node but it's character driver so the page cache doesn't have it. That's why we encounter BUG_ON in radix tree operation. Anyway, to test, I implemented set_page_dirty in my dummy driver.
Okay and the above test_set_page_dirty() example is sufficient ?
I guess just return 0 is sufficeint without any dirting a page.
With only that, it doesn't work because I need to modify migrate.c to work non-lru-mapped-page and changing PG_isolated flag which is override of PG_reclaim which is cleared in set_page_dirty.
Got it, so what changes you did ? Implemented PG_isolated differently not by overriding PG_reclaim or something else ? Yes set_page_dirty indeed clears the PG_reclaim flag.
With that, it seems to work. But I'm not saying it's right model now
So the mapped pages migration was successful ? Even after overloading filp->f_mapping->a_ops = &test_aops, we still have the RMAP information intact with filp->f_mappinp pointed interval tree. But would really like to see the code changes.
for device drivers. In runtime, replacing filp->f_mapping->a_ops with custom a_ops of own driver seems to be hacky to me.
Yeah I thought so.
So, I'm considering now new pseudo fs "movable_inode" which will support
struct file *movable_inode_getfile(const char *name, const struct file_operations *fop, const struct address_space_operations *a_ops) { struct path path; struct qstr this; struct inode *inode; struct super_block *sb;
this.name = name; this.len = strlen(name); this.hash = 0; sb = movable_mnt.mnt_sb; patch.denty = d_alloc_pseudo(movable_inode_mnt->mnt_sb, &this); patch.mnt = mntget(movable_inode_mnt); inode = new_inode(sb); .. .. inode->i_mapping->a_ops = a_ops; d_instantiate(path.dentry, inode); return alloc_file(&path, FMODE_WRITE | FMODE_READ, f_op);}
And in our driver, we can change vma->vm_file with new one.
int test_mmap(struct file *filp, struct vm_area_structd *vma) { struct file *newfile = movable_inode_getfile("[test"], filep->f_op, &test_aops); vma->vm_file = newfile; .. .. }
When I read mmap_region in mm/mmap.c, it's reasonable usecase which dirver's mmap changes vma->vm_file with own file.
I will look into these details.
Anyway, it needs many subtle changes in mm/vfs/driver side so need to review from each maintainers related subsystem so I want to not be hurry.
Sure, makes sense. Mean while it will be really great if you could share your code changes as described above, so that I can try them out.
It's almost done for draft version and I'm doing stress test now and fortunately, doesn't see the problem until now.
I will send you when I'm ready.
Thanks.