Messages in this thread | | | Date | Sun, 15 Aug 2021 04:31:00 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH v14 071/138] mm/writeback: Add filemap_dirty_folio() |
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On Thu, Aug 12, 2021 at 06:07:05PM +0200, Vlastimil Babka wrote: > On 7/15/21 5:35 AM, Matthew Wilcox (Oracle) wrote: > > Reimplement __set_page_dirty_nobuffers() as a wrapper around > > filemap_dirty_folio(). > > I assume it becomes obvious later why the new "mapping" parameter instead of > taking it from the folio, but maybe the changelog should say it here?
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mm/writeback: Add filemap_dirty_folio()
Reimplement __set_page_dirty_nobuffers() as a wrapper around filemap_dirty_folio(). Eventually folio_mark_dirty() will pass the folio's mapping to the address space's ->dirty_folio() operation, so add the parameter to filemap_dirty_folio() now.
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Nobody seems quite sure whether it's possible to truncate (or otherwise remove) a page from a file while it's being marked as dirty. viz:
int set_page_dirty(struct page *page) { struct address_space *mapping = page_mapping(page); if (likely(mapping)) { ... return mapping->a_ops->set_page_dirty(page); }
so ->set_page_dirty can only be called if page has a mapping (obviously, otherwise we wouldn't know whose ->set_page_dirty to call). But then in __set_page_dirty_nobuffers(), we check to see if mapping has become unset:
if (!TestSetPageDirty(page)) { struct address_space *mapping = page_mapping(page);
if (!mapping) { unlock_page_memcg(page); return 1; }
Confusingly, the comment to __set_page_dirty_nobuffers says:
* The caller must ensure this doesn't race with truncation. Most will simply * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and * the pte lock held, which also locks out truncation.
I believe this is left-over from commit 2d6d7f982846 in 2015.
Anyway, passing mapping as a parameter is something we already do for just about every other address_space operation, and we already called page_mapping() to get it, so why make the callee call it again? Not to mention people get confused about whether to call page_mapping() or just look at page->mapping. Changing the ->set_page_dirty() operation to ->dirty_folio() is something I've postponed until the 5.17/5.18 timeframe, but we might as well pass the parameter to filemap_dirty_folio() now.
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