lkml.org 
[lkml]   [2023]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC 2/2] shmem: add support to ignore swap
    On Wed, Feb 22, 2023 at 05:04:32PM -0800, Yosry Ahmed wrote:
    > On Wed, Feb 22, 2023 at 4:53 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
    > >
    > > On Wed, Feb 08, 2023 at 12:33:37PM -0800, Yosry Ahmed wrote:
    > > > On Wed, Feb 8, 2023 at 9:45 AM Matthew Wilcox <willy@infradead.org> wrote:
    > > > >
    > > > > On Wed, Feb 08, 2023 at 08:01:01AM -0800, Luis Chamberlain wrote:
    > > > > > On Tue, Feb 07, 2023 at 04:01:51AM +0000, Matthew Wilcox wrote:
    > > > > > > On Mon, Feb 06, 2023 at 06:52:59PM -0800, Luis Chamberlain wrote:
    > > > > > > > @@ -1334,11 +1336,15 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
    > > > > > > > struct shmem_inode_info *info;
    > > > > > > > struct address_space *mapping = folio->mapping;
    > > > > > > > struct inode *inode = mapping->host;
    > > > > > > > + struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
    > > > > > > > swp_entry_t swap;
    > > > > > > > pgoff_t index;
    > > > > > > >
    > > > > > > > BUG_ON(!folio_test_locked(folio));
    > > > > > > >
    > > > > > > > + if (wbc->for_reclaim && unlikely(sbinfo->noswap))
    > > > > > > > + return AOP_WRITEPAGE_ACTIVATE;
    > > > > > >
    > > > > > > Not sure this is the best way to handle this. We'll still incur the
    > > > > > > oevrhead of tracking shmem pages on the LRU, only to fail to write them
    > > > > > > out when the VM thinks we should get rid of them. We'd be better off
    > > > > > > not putting them on the LRU in the first place.
    > > > > >
    > > > > > Ah, makes sense, so in effect then if we do that then on reclaim
    > > > > > we should be able to even WARN_ON(sbinfo->noswap) assuming we did
    > > > > > everthing right.
    > > > > >
    > > > > > Hrm, we have invalidate_mapping_pages(mapping, 0, -1) but that seems a bit
    > > > > > too late how about d_mark_dontcache() on shmem_get_inode() instead?
    > > > >
    > > > > I was thinking that the two calls to folio_add_lru() in mm/shmem.c
    > > > > should be conditional on sbinfo->noswap.
    > > > >
    > > >
    > > > Wouldn't this cause the folio to not show up in any lru lists, even
    > > > the unevictable one, which may be a strange discrepancy?
    > > >
    > > > Perhaps we can do something like shmem_lock(), which calls
    > > > mapping_set_unevictable(), which will make folio_evictable() return
    > > > true and the LRUs code will take care of the rest?
    > >
    > > If shmem_lock() should take care of that is that because writepages()
    > > should not happen or because we have that info->flags & VM_LOCKED stop
    > > gap on writepages()? If the earlier then shouldn't we WARN_ON_ONCE()
    > > if writepages() is called on info->flags & VM_LOCKED?
    > >
    > > While I see the value in mapping_set_unevictable() I am not sure I see
    > > the point in using shmem_lock(). I don't see why we should constrain
    > > noswap tmpfs option to RLIMIT_MEMLOCK
    > >
    > > Please correct me if I'm wrong but the limit seem to be designed for
    > > files / IPC / unprivileged perf limits. On the contrary, we'd bump the
    > > count for each new inode. Using shmem_lock() would also complicate the
    > > inode allocation on shmem as we'd have to unwind on failure from the
    > > user_shm_lock(). It would also beg the question of when to capture a
    > > ucount for an inode, should we just share one for the superblock at
    > > shmem_fill_super() or do we really need to capture it at every single
    > > inode creation? In theory we could end up with different limits.
    > >
    > > So why not just use mapping_set_unevictable() alone for this use case?
    >
    > Sorry if I wasn't clear, I did NOT mean that we should use
    > shmem_lock(), I meant that we do something similar to what
    > shmem_lock() does and use mapping_set_unevictable() or similar.

    Ah OK! Sure yeah I reviewed shmem_lock() usage and I don't think it
    and its rtlimit baggage makes sense here so the only thing to do is
    just mapping_set_unevictable().

    > I think we just need to make sure that if we use
    > mapping_set_unevictable() does not imply that shmem_lock() was used
    > (i.e no code assumes that if the shmem mapping is unevictable then
    > shmem_lock() was used).

    The *other* stuff that shmem_lock() does is rlimit rlimit related
    to RLIMIT_MEMLOCK, I can't think off hand why we'd confuse the two
    use cases at the moment, but I'll give it another good luck with this
    in mind.

    I'll test what I have and post a v2 with the feedback received.

    Thanks,

    Luis

    \
     
     \ /
      Last update: 2023-03-27 00:33    [W:3.252 / U:0.212 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site